⇅ Sequence Features

Sequence Features Preprocessing

Sequence features are transformed into an integer valued matrix of size n x l (where n is the number of rows and l is the minimum of the length of the longest sequence and a max_sequence_length parameter) and added to HDF5 with a key that reflects the name of column in the dataset. Each sequence in mapped to a list of integers internally. First, a tokenizer converts each sequence to a list of tokens (default tokenization is done by splitting on spaces). Next, a dictionary is constructed which maps each unique token to its frequency in the dataset column. Tokens are ranked by frequency and a sequential integer ID is assigned from the most frequent to the most rare. Ludwig uses <PAD>, <UNK>, <SOS>, and <EOS> special symbols for padding, unknown, start, and end, consistent with common NLP deep learning practices. Special symbols can also be set manually in the preprocessing config. The column name is added to the JSON file, with an associated dictionary containing

1. the mapping from integer to string (idx2str)
2. the mapping from string to id (str2idx)
3. the mapping from string to frequency (str2freq)
4. the maximum length of all sequences (max_sequence_length)
5. additional preprocessing information (how to fill missing values and what token to use to fill missing values)

The parameters available for preprocessing are

• tokenizer (default space): defines how to map from the raw string content of the dataset column to a sequence of elements. For the available options refer to the Tokenizers section.
• vocab_file (default null) filepath string to a UTF-8 encoded file containing the sequence's vocabulary. On each line the first string until \t or \n is considered a word.
• max_sequence_length (default 256): the maximum length of the sequence. Sequences that are longer than this value will be truncated, while sequences that are shorter will be padded.
• most_common (default 20000): the maximum number of most common tokens to be considered. if the data contains more than this amount, the most infrequent tokens will be treated as unknown.
• padding_symbol (default <PAD>): the string used as a padding symbol, mapped to the integer ID 0 in the vocabulary.
• unknown_symbol (default <UNK>): the string used as the unknown placeholder, mapped to the integer ID 1 in the vocabulary.
• padding (default right): the direction of the padding. right and left are available options.
• lowercase (default false): If true, converts the string to lowercase before tokenizing.
• missing_value_strategy (default fill_with_const): what strategy to follow when there's a missing value in the column. The value should be one of fill_with_const (replaces the missing value with the value specified by the fill_value parameter), fill_with_mode (replaces the missing values with the most frequent value in the column), bfill (replaces the missing values with the next valid value), ffill (replaces the missing values with the previous valid value) or drop_row.
• fill_value (default <UNK>): the value to replace the missing values with in case the missing_value_strategy is fill_value.

Configuration example:

name: sequence_column_name
type: sequence
preprocessing:
       tokenizer: space
       vocab_file: null
       max_sequence_length: 256
       most_common: 20000
       padding_symbol: <PAD>
       unknown_symbol: <UNK>
       padding: right
       lowercase: false
       missing_value_strategy: fill_with_const
       fill_value: <UNK>

Preprocessing parameters can also be defined once and applied to all sequence input features using the Type-Global Preprocessing section.

Sequence Input Features and Encoders

Sequence features have several encoders and each of them has its own parameters. Inputs are of size b while outputs are of size b x h where b is the batch size and h is the dimensionality of the output of the encoder. In case a representation for each element of the sequence is needed (for example for tagging them, or for using an attention mechanism), one can specify the parameter reduce_output to be null and the output will be a b x s x h tensor where s is the length of the sequence. Some encoders, because of their inner workings, may require additional parameters to be specified in order to obtain one representation for each element of the sequence. For instance the parallel_cnn encoder by default pools and flattens the sequence dimension and then passes the flattened vector through fully connected layers, so in order to obtain the full sequence tensor one has to specify reduce_output: null.

The encoder parameters specified at the feature level are:

• tied (default null): name of another input feature to tie the weights of the encoder with. It needs to be the name of a feature of the same type and with the same encoder parameters.

Example sequence feature entry in the input features list:

name: sequence_column_name
type: sequence
tied: null
encoder: 
    type: stacked_cnn

The available encoder parameters:

• type (default parallel_cnn): the name of the encoder to use to encode the sequence, one of embed, parallel_cnn, stacked_cnn, stacked_parallel_cnn, rnn, cnnrnn, transformer and passthrough (equivalent to null or None).

Encoder type and encoder parameters can also be defined once and applied to all sequence input features using the Type-Global Encoder section.

Embed Encoder

The embed encoder simply maps each integer in the sequence to an embedding, creating a b x s x h tensor where b is the batch size, s is the length of the sequence and h is the embedding size. The tensor is reduced along the s dimension to obtain a single vector of size h for each element of the batch. If you want to output the full b x s x h tensor, you can specify reduce_output: null.

       +------+
       |Emb 12|
       +------+
+--+   |Emb 7 |
|12|   +------+
|7 |   |Emb 43|   +-----------+
|43|   +------+   |Aggregation|
|65+--->Emb 65+--->Reduce     +-->
|23|   +------+   |Operation  |
|4 |   |Emb 23|   +-----------+
|1 |   +------+
+--+   |Emb 4 |
       +------+
       |Emb 1 |
       +------+

These are the parameters available for the embed encoder

• representation (default dense): the possible values are dense and sparse. dense means the embeddings are initialized randomly, sparse means they are initialized to be one-hot encodings.
• embedding_size (default 256): it is the maximum embedding size, the actual size will be min(vocabulary_size, embedding_size) for dense representations and exactly vocabulary_size for the sparse encoding, where vocabulary_size is the number of unique strings appearing in the training set input column plus the number of special tokens (<UNK>, <PAD>, <SOS>, <EOS>).
• embeddings_trainable (default true): If true embeddings are trained during the training process, if false embeddings are fixed. It may be useful when loading pretrained embeddings for avoiding finetuning them. This parameter has effect only when representation is dense, sparse one-hot encodings are not trainable.
• pretrained_embeddings (default null): by default dense embeddings are initialized randomly, but this parameter allows to specify a path to a file containing embeddings in the GloVe format. When the file containing the embeddings is loaded, only the embeddings with labels present in the vocabulary are kept, the others are discarded. If the vocabulary contains strings that have no match in the embeddings file, their embeddings are initialized with the average of all other embedding plus some random noise to make them different from each other. This parameter has effect only if representation is dense.
• embeddings_on_cpu (default false): by default embedding matrices are stored on GPU memory if a GPU is used, as it allows for faster access, but in some cases the embedding matrix may be too large. This parameter forces the placement of the embedding matrix in regular memory and the CPU is used for embedding lookup, slightly slowing down the process as a result of data transfer between CPU and GPU memory.
• dropout (default 0): dropout rate.
• weights_initializer (default glorot_uniform): initializer for the weight matrix. Options are: constant, identity, zeros, ones, orthogonal, normal, uniform, truncated_normal, variance_scaling, glorot_normal, glorot_uniform, xavier_normal, xavier_uniform, he_normal, he_uniform, lecun_normal, lecun_uniform. Alternatively it is possible to specify a dictionary with a key type that identifies the type of initializer and other keys for its parameters, e.g. {type: normal, mean: 0, stddev: 0}. To know the parameters of each initializer, please refer to torch.nn.init.
• reduce_output (default sum): defines how to reduce the output tensor along the s sequence length dimension if the rank of the tensor is greater than 2. Available values are: sum, mean or avg, max, concat (concatenates along the sequence dimension), last (selects the last vector of the sequence dimension) and null (which does not reduce and returns the full tensor).

Example sequence feature entry in the input features list using an embed encoder:

name: sequence_column_name
type: sequence
encoder: 
    type: embed
    representation: dense
    embedding_size: 256
    embeddings_trainable: true
    embeddings_on_cpu: false
    dropout: 0
    reduce_output: sum

Parallel CNN Encoder

The parallel cnn encoder is inspired by Yoon Kim's Convolutional Neural Network for Sentence Classification. It works by first mapping the input integer sequence b x s (where b is the batch size and s is the length of the sequence) into a sequence of embeddings, then it passes the embedding through a number of parallel 1d convolutional layers with different filter size (by default 4 layers with filter size 2, 3, 4 and 5), followed by max pooling and concatenation. This single vector concatenating the outputs of the parallel convolutional layers is then passed through a stack of fully connected layers and returned as a b x h tensor where h is the output size of the last fully connected layer. If you want to output the full b x s x h tensor, you can specify reduce_output: null.

                    +-------+   +----+
                 +-->1D Conv+--->Pool+--+
       +------+  |  |Width 2|   +----+  |
       |Emb 12|  |  +-------+           |
       +------+  |                      |
+--+   |Emb 7 |  |  +-------+   +----+  |
|12|   +------+  +-->1D Conv+--->Pool+--+
|7 |   |Emb 43|  |  |Width 3|   +----+  |            +---------+
|43|   +------+  |  +-------+           |  +------+  |Fully    |
|65+-->Emb 65 +--+                      +-->Concat+-->Connected+-->
|23|   +------+  |  +-------+   +----+  |  +------+  |Layers   |
|4 |   |Emb 23|  +-->1D Conv+--->Pool+--+            +---------+
|1 |   +------+  |  |Width 4|   +----+  |
+--+   |Emb 4 |  |  +-------+           |
       +------+  |                      |
       |Emb 1 |  |  +-------+   +----+  |
       +------+  +-->1D Conv+--->Pool+--+
                    |Width 5|   +----+
                    +-------+

These are the available parameters for a parallel cnn encoder:

• representation (default dense): the possible values are dense and sparse. dense means the embeddings are initialized randomly, sparse means they are initialized to be one-hot encodings.
• embedding_size (default 256): it is the maximum embedding size, the actual size will be min(vocabulary_size, embedding_size) for dense representations and exactly vocabulary_size for the sparse encoding, where vocabulary_size is the number of unique strings appearing in the training set input column plus the number of special tokens (<UNK>, <PAD>, <SOS>, <EOS>).
• embeddings_trainable (default true): If true embeddings are trained during the training process, if false embeddings are fixed. It may be useful when loading pretrained embeddings for avoiding finetuning them. This parameter has effect only when representation is dense as sparse one-hot encodings are not trainable.
• pretrained_embeddings (default null): by default dense embeddings are initialized randomly, but this parameter allows to specify a path to a file containing embeddings in the GloVe format. When the file containing the embeddings is loaded, only the embeddings with labels present in the vocabulary are kept, the others are discarded. If the vocabulary contains strings that have no match in the embeddings file, their embeddings are initialized with the average of all other embedding plus some random noise to make them different from each other. This parameter has effect only if representation is dense.
• embeddings_on_cpu (default false): by default embedding matrices are stored on GPU memory if a GPU is used, as it allows for faster access, but in some cases the embedding matrix may be too large. This parameter forces the placement of the embedding matrix in regular memory and the CPU is used for embedding lookup, slightly slowing down the process as a result of data transfer between CPU and GPU memory.
• conv_layers (default null): a list of dictionaries containing the parameters of all the convolutional layers. The length of the list determines the number of parallel convolutional layers and the content of each dictionary determines the parameters for a specific layer. The available parameters for each layer are: activation, dropout, norm, norm_params, num_filters, filter_size, strides, padding, dilation_rate, use_bias, pool_function, pool_padding, pool_size, pool_strides, bias_initializer, weights_initializer. If any of those values is missing from the dictionary, the default one specified as a parameter of the encoder will be used instead. If both conv_layers and num_conv_layers are null, a default list will be assigned to conv_layers with the value [{filter_size: 2}, {filter_size: 3}, {filter_size: 4}, {filter_size: 5}].
• num_conv_layers (default null): if conv_layers is null, this is the number of parallel convolutional layers.
• filter_size (default 3): if a filter_size is not already specified in conv_layers this is the default filter_size that will be used for each layer. It indicates how wide is the 1d convolutional filter.
• num_filters (default 256): if a num_filters is not already specified in conv_layers this is the default num_filters that will be used for each layer. It indicates the number of filters, and by consequence the output channels of the 1d convolution.
• pool_function (default max): pooling function: max will select the maximum value. Any of average, avg or mean will compute the mean value.
• pool_size (default null): if a pool_size is not already specified in conv_layers this is the default pool_size that will be used for each layer. It indicates the size of the max pooling that will be performed along the s sequence dimension after the convolution operation.
• fc_layers (default null): a list of dictionaries containing the parameters of all the fully connected layers. The length of the list determines the number of stacked fully connected layers and the content of each dictionary determines the parameters for a specific layer. The available parameters for each layer are: activation, dropout, norm, norm_params, output_size, use_bias, bias_initializer and weights_initializer. If any of those values is missing from the dictionary, the default one specified as a parameter of the encoder will be used instead. If both fc_layers and num_fc_layers are null, a default list will be assigned to fc_layers with the value [{output_size: 512}, {output_size: 256}] (only applies if reduce_output is not null).
• num_fc_layers (default null): if fc_layers is null, this is the number of stacked fully connected layers (only applies if reduce_output is not null).
• output_size (default 256): if output_size is not already specified in fc_layers this is the default output_size that will be used for each layer. It indicates the size of the output of a fully connected layer.
• use_bias (default true): boolean, whether the layer uses a bias vector.
• weights_initializer (default glorot_uniform): initializer for the weights matrix. Options are: constant, identity, zeros, ones, orthogonal, normal, uniform, truncated_normal, variance_scaling, glorot_normal, glorot_uniform, xavier_normal, xavier_uniform, he_normal, he_uniform, lecun_normal, lecun_uniform. Alternatively it is possible to specify a dictionary with a key type that identifies the type of initializer and other keys for its parameters, e.g. {type: normal, mean: 0, stddev: 0}. To know the parameters of each initializer, please refer to torch.nn.init.
• bias_initializer (default zeros): initializer for the bias vector. Options are: constant, identity, zeros, ones, orthogonal, normal, uniform, truncated_normal, variance_scaling, glorot_normal, glorot_uniform, xavier_normal, xavier_uniform, he_normal, he_uniform, lecun_normal, lecun_uniform. Alternatively it is possible to specify a dictionary with a key type that identifies the type of initializer and other keys for its parameters, e.g. {type: normal, mean: 0, stddev: 0}. To know the parameters of each initializer, please refer to torch.nn.init.
• norm (default null): if a norm is not already specified in fc_layers this is the default norm that will be used for each layer. It indicates how the output should be normalized and may be one of null, batch or layer.
• norm_params (default null): parameters used if norm is either batch or layer. For information on parameters used with batch see the Torch documentation on batch normalization or for layer see the Torch documentation on layer normalization.
• activation (default relu): if an activation is not already specified in fc_layers this is the default activation that will be used for each layer. It indicates the activation function applied to the output.
• dropout (default 0): dropout rate
• reduce_output (default sum): defines how to reduce the output tensor along the s sequence length dimension if the rank of the tensor is greater than 2. Available values are: sum, mean or avg, max, concat (concatenates along the sequence dimension), last (selects the last vector of the sequence dimension) and null (which does not reduce and returns the full tensor).

Example sequence feature entry in the input features list using a parallel cnn encoder:

name: sequence_column_name
type: sequence
encoder: 
    type: parallel_cnn
    representation: dense
    embedding_size: 256
    embeddings_trainable: true
    filter_size: 3
    num_filters: 256
    pool_function: max
    output_size: 256
    use_bias: true
    weights_initializer: glorot_uniform
    bias_initializer: zeros
    activation: relu
    dropout: 0.0
    reduce_output: sum

Stacked CNN Encoder

The stacked cnn encoder is inspired by Xiang Zhang at all's Character-level Convolutional Networks for Text Classification. It works by first mapping the input integer sequence b x s (where b is the batch size and s is the length of the sequence) into a sequence of embeddings, then it passes the embedding through a stack of 1d convolutional layers with different filter size (by default 6 layers with filter size 7, 7, 3, 3, 3 and 3), followed by an optional final pool and by a flatten operation. This single flatten vector is then passed through a stack of fully connected layers and returned as a b x h tensor where h is the output size of the last fully connected layer. If you want to output the full b x s x h tensor, you can specify the pool_size of all your conv_layers to be null and reduce_output: null, while if pool_size has a value different from null and reduce_output: null the returned tensor will be of shape b x s' x h, where s' is width of the output of the last convolutional layer.

       +------+
       |Emb 12|
       +------+
+--+   |Emb 7 |
|12|   +------+
|7 |   |Emb 43|   +----------------+   +---------+
|43|   +------+   |1D Conv         |   |Fully    |
|65+--->Emb 65+--->Layers          +--->Connected+-->
|23|   +------+   |Different Widths|   |Layers   |
|4 |   |Emb 23|   +----------------+   +---------+
|1 |   +------+
+--+   |Emb 4 |
       +------+
       |Emb 1 |
       +------+

These are the parameters available for the stack cnn encoder:

• representation (default dense): the possible values are dense and sparse. dense means the embeddings are initialized randomly, sparse means they are initialized to be one-hot encodings.
• embedding_size (default 256): the maximum embedding size, the actual size will be min(vocabulary_size, embedding_size) for dense representations and exactly vocabulary_size for the sparse encoding, where vocabulary_size is the number of unique strings appearing in the training set input column plus the number of special tokens (<UNK>, <PAD>, <SOS>, <EOS>).
• embeddings_trainable (default true): If true embeddings are trained during the training process, if false embeddings are fixed. It may be useful when loading pretrained embeddings for avoiding finetuning them. This parameter has effect only when representation is dense as sparse one-hot encodings are not trainable.
• pretrained_embeddings (default null): by default dense embeddings are initialized randomly, but this parameter allows to specify a path to a file containing embeddings in the GloVe format. When the file containing the embeddings is loaded, only the embeddings with labels present in the vocabulary are kept, the others are discarded. If the vocabulary contains strings that have no match in the embeddings file, their embeddings are initialized with the average of all other embedding plus some random noise to make them different from each other. This parameter has effect only if representation is dense.
• embeddings_on_cpu (default false): by default embedding matrices are stored on GPU memory if a GPU is used, as it allows for faster access, but in some cases the embedding matrix may be too large. This parameter forces the placement of the embedding matrix in regular memory and the CPU is used for embedding lookup, slightly slowing down the process as a result of data transfer between CPU and GPU memory.
• conv_layers (default null): a list of dictionaries containing the parameters of all the convolutional layers. The length of the list determines the number of stacked convolutional layers and the content of each dictionary determines the parameters for a specific layer. The available parameters for each layer are: activation, dropout, norm, norm_params, num_filters, filter_size, strides, padding, dilation_rate, use_bias, pool_function, pool_padding, pool_size, pool_strides, bias_initializer, weights_initializer. If any of those values is missing from the dictionary, the default one specified as a parameter of the encoder will be used instead. If both conv_layers and num_conv_layers are null, a default list will be assigned to conv_layers with the value [{filter_size: 7, pool_size: 3}, {filter_size: 7, pool_size: 3}, {filter_size: 3, pool_size: null}, {filter_size: 3, pool_size: null}, {filter_size: 3, pool_size: null}, {filter_size: 3, pool_size: 3}].
• num_conv_layers (default null): if conv_layers is null, this is the number of stacked convolutional layers.
• filter_size (default 3): if a filter_size is not already specified in conv_layers this is the default filter_size that will be used for each layer. It indicates how wide is the 1d convolutional filter.
• num_filters (default 256): if a num_filters is not already specified in conv_layers this is the default num_filters that will be used for each layer. It indicates the number of filters, and by consequence the output channels of the 1d convolution.
• strides (default 1): stride length of the convolution
• padding (default same): one of valid or same.
• dilation_rate (default 1): dilation rate to use for dilated convolution
• pool_function (default max): pooling function: max will select the maximum value. Any of average, avg or mean will compute the mean value.
• pool_size (default null): if a pool_size is not already specified in conv_layers this is the default pool_size that will be used for each layer. It indicates the size of the max pooling that will be performed along the s sequence dimension after the convolution operation.
• pool_strides (default null): factor to scale down
• pool_padding (default same): one of valid or same
• fc_layers (default null): a list of dictionaries containing the parameters of all the fully connected layers. The length of the list determines the number of stacked fully connected layers and the content of each dictionary determines the parameters for a specific layer. The available parameters for each layer are: activation, dropout, norm, norm_params, output_size, use_bias, bias_initializer and weights_initializer. If any of those values is missing from the dictionary, the default one specified as a parameter of the encoder will be used instead. If both fc_layers and num_fc_layers are null, a default list will be assigned to fc_layers with the value [{output_size: 512}, {output_size: 256}] (only applies if reduce_output is not null).
• num_fc_layers (default null): if fc_layers is null, this is the number of stacked fully connected layers (only applies if reduce_output is not null).
• output_size (default 256): if an output_size is not already specified in fc_layers this is the default output_size that will be used for each layer. It indicates the size of the output of a fully connected layer.
• use_bias (default true): boolean, whether the layer uses a bias vector.
• weights_initializer (default glorot_uniform): initializer for the weight matrix. Options are: constant, identity, zeros, ones, orthogonal, normal, uniform, truncated_normal, variance_scaling, glorot_normal, glorot_uniform, xavier_normal, xavier_uniform, he_normal, he_uniform, lecun_normal, lecun_uniform. Alternatively it is possible to specify a dictionary with a key type that identifies the type of initializer and other keys for its parameters, e.g. {type: normal, mean: 0, stddev: 0}. To know the parameters of each initializer, please refer to torch.nn.init.
• bias_initializer (default zeros): initializer for the bias vector. Options are: constant, identity, zeros, ones, orthogonal, normal, uniform, truncated_normal, variance_scaling, glorot_normal, glorot_uniform, xavier_normal, xavier_uniform, he_normal, he_uniform, lecun_normal, lecun_uniform. Alternatively it is possible to specify a dictionary with a key type that identifies the type of initializer and other keys for its parameters, e.g. {type: normal, mean: 0, stddev: 0}. To know the parameters of each initializer, please refer to torch.nn.init.
• norm (default null): if a norm is not already specified in fc_layers this is the default norm that will be used for each layer. It indicates how the output should be normalized and may be one of null, batch or layer.
• norm_params (default null): parameters used if norm is either batch or layer. For information on parameters used with batch see the Torch documentation on batch normalization or for layer see the Torch documentation on layer normalization.
• activation (default relu): if an activation is not already specified in fc_layers this is the default activation that will be used for each layer. It indicates the activation function applied to the output.
• dropout (default 0): dropout rate
• reduce_output (default max): defines how to reduce the output tensor of the convolutional layers along the s sequence length dimension if the rank of the tensor is greater than 2. Available values are: sum, mean or avg, max, concat (concatenates along the sequence dimension), last (returns the last vector of the sequence dimension) and null (which does not reduce and returns the full tensor).

Example sequence feature entry in the input features list using a parallel cnn encoder:

name: sequence_column_name
type: sequence
encoder: 
    type: stacked_cnn
    representation: dense
    embedding_size: 256
    embeddings_trainable: true
    filter_size: 3
    num_filters: 256
    strides: 1
    padding: same
    dilation_rate: 1
    pool_function: max
    pool_padding: same
    output_size: 256
    use_bias: true
    weights_initializer: glorot_uniform
    bias_initializer: zeros
    activation: relu
    dropout: 0
    reduce_output: max

Stacked Parallel CNN Encoder

The stacked parallel cnn encoder is a combination of the Parallel CNN and the Stacked CNN encoders where each layer of the stack is composed of parallel convolutional layers. It works by first mapping the input integer sequence b x s (where b is the batch size and s is the length of the sequence) into a sequence of embeddings, then it passes the embedding through a stack of several parallel 1d convolutional layers with different filter size, followed by an optional final pool and by a flatten operation. This single flattened vector is then passed through a stack of fully connected layers and returned as a b x h tensor where h is the output size of the last fully connected layer. If you want to output the full b x s x h tensor, you can specify reduce_output: null.

                  +-------+                     +-------+
                +->1D Conv+-+                 +->1D Conv+-+
      +------+  | |Width 2| |                 | |Width 2| |
      |Emb 12|  | +-------+ |                 | +-------+ |
      +------+  |           |                 |           |
+--+  |Emb 7 |  | +-------+ |                 | +-------+ |
|12|  +------+  +->1D Conv+-+                 +->1D Conv+-+
|7 |  |Emb 43|  | |Width 3| |                 | |Width 3| |                 +---------+
|43|  +------+  | +-------+ | +------+  +---+ | +-------+ | +------+ +----+ |Fully    |
|65+->Emb 65 +--+           +->Concat+-->...+-+           +->Concat+->Pool+->Connected+-->
|23|  +------+  | +-------+ | +------+  +---+ | +-------+ | +------+ +----+ |Layers   |
|4 |  |Emb 23|  +->1D Conv+-+                 +->1D Conv+-+                 +---------+
|1 |  +------+  | |Width 4| |                 | |Width 4| |
+--+  |Emb 4 |  | +-------+ |                 | +-------+ |
      +------+  |           |                 |           |
      |Emb 1 |  | +-------+ |                 | +-------+ |
      +------+  +->1D Conv+-+                 +->1D Conv+-+
                  |Width 5|                     |Width 5|
                  +-------+                     +-------+

These are the available parameters for the stack parallel cnn encoder:

• representation (default dense): the possible values are dense and sparse. dense means the embeddings are initialized randomly, sparse means they are initialized to be one-hot encodings.
• embedding_size (default 256): the maximum embedding size, the actual size will be min(vocabulary_size, embedding_size) for dense representations and exactly vocabulary_size for the sparse encoding, where vocabulary_size is the number of unique strings appearing in the training set input column plus the number of special tokens (<UNK>, <PAD>, <SOS>, <EOS>).
• embeddings_trainable (default true): If true embeddings are trained during the training process, if false embeddings are fixed. It may be useful when loading pretrained embeddings for avoiding finetuning them. This parameter has effect only when representation is dense as sparse one-hot encodings are not trainable.
• pretrained_embeddings (default null): by default dense embeddings are initialized randomly, but this parameter allows to specify a path to a file containing embeddings in the GloVe format. When the file containing the embeddings is loaded, only the embeddings with labels present in the vocabulary are kept, the others are discarded. If the vocabulary contains strings that have no match in the embeddings file, their embeddings are initialized with the average of all other embedding plus some random noise to make them different from each other. This parameter has effect only if representation is dense.
• embeddings_on_cpu (default false): by default embedding matrices are stored on GPU memory if a GPU is used, as it allows for faster access, but in some cases the embedding matrix may be too large. This parameter forces the placement of the embedding matrix in regular memory and the CPU is used for embedding lookup, slightly slowing down the process as a result of data transfer between CPU and GPU memory.
• stacked_layers (default null): a nested list of lists of dictionaries containing the parameters of the stack of parallel convolutional layers. The length of the list determines the number of stacked parallel convolutional layers, length of the sub-lists determines the number of parallel conv layers and the content of each dictionary determines the parameters for a specific layer. The available parameters for each layer are: activation, dropout, norm, norm_params, num_filters, filter_size, strides, padding, dilation_rate, use_bias, pool_function, pool_padding, pool_size, pool_strides, bias_initializer, weights_initializer. If any of those values is missing from the dictionary, the default one specified as a parameter of the encoder will be used instead. If both stacked_layers and num_stacked_layers are null, a default list will be assigned to stacked_layers with the value [[{filter_size: 2}, {filter_size: 3}, {filter_size: 4}, {filter_size: 5}], [{filter_size: 2}, {filter_size: 3}, {filter_size: 4}, {filter_size: 5}], [{filter_size: 2}, {filter_size: 3}, {filter_size: 4}, {filter_size: 5}]].
• num_stacked_layers (default null): if stacked_layers is null, this is the number of elements in the stack of parallel convolutional layers.
• filter_size (default 3): if a filter_size is not already specified in stacked_layers this is the default filter_size that will be used for each layer. It indicates how wide is the 1d convolutional filter.
• num_filters (default 256): if a num_filters is not already specified in stacker_layers this is the default num_filters that will be used for each layer. It indicates the number of filters, and by consequence the output channels of the 1d convolution.
• pool_function (default max): pooling function: max will select the maximum value. Any of average, avg or mean will compute the mean value.
• pool_size (default null): if a pool_size is not already specified in stacked_layers this is the default pool_size that will be used for each layer. It indicates the size of the max pooling that will be performed along the s sequence dimension after the convolution operation.
• fc_layers (default null): a list of dictionaries containing the parameters of all the fully connected layers. The length of the list determines the number of stacked fully connected layers and the content of each dictionary determines the parameters for a specific layer. The available parameters for each layer are: activation, dropout, norm, norm_params, output_size, use_bias, bias_initializer and weights_initializer. If any of those values is missing from the dictionary, the default one specified as a parameter of the encoder will be used instead. If both fc_layers and num_fc_layers are null, a default list will be assigned to fc_layers with the value [{output_size: 512}, {output_size: 256}] (only applies if reduce_output is not null).
• num_fc_layers (default null): if fc_layers is null, this is the number of stacked fully connected layers (only applies if reduce_output is not null).
• output_size (default 256): if an output_size is not already specified in fc_layers this is the default output_size that will be used for each layer. It indicates the size of the output of a fully connected layer.
• use_bias (default true): boolean, whether the layer uses a bias vector.
• weights_initializer (default glorot_uniform): initializer for the weights matrix. Options are: constant, identity, zeros, ones, orthogonal, normal, uniform, truncated_normal, variance_scaling, glorot_normal, glorot_uniform, xavier_normal, xavier_uniform, he_normal, he_uniform, lecun_normal, lecun_uniform. Alternatively it is possible to specify a dictionary with a key type that identifies the type of initializer and other keys for its parameters, e.g. {type: normal, mean: 0, stddev: 0}. To know the parameters of each initializer, please refer to torch.nn.init.
• bias_initializer (default zeros): initializer for the bias vector. Options are: constant, identity, zeros, ones, orthogonal, normal, uniform, truncated_normal, variance_scaling, glorot_normal, glorot_uniform, xavier_normal, xavier_uniform, he_normal, he_uniform, lecun_normal, lecun_uniform. Alternatively it is possible to specify a dictionary with a key type that identifies the type of initializer and other keys for its parameters, e.g. {type: normal, mean: 0, stddev: 0}. To know the parameters of each initializer, please refer to torch.nn.init.
• norm (default null): if a norm is not already specified in fc_layers this is the default norm that will be used for each layer. It indicates how the output should be normalized and may be one of null, batch or layer.
• norm_params (default null): parameters used if norm is either batch or layer. For information on parameters used with batch see the Torch documentation on batch normalization or for layer see the Torch documentation on layer normalization.
• activation (default relu): if an activation is not already specified in fc_layers this is the default activation that will be used for each layer. It indicates the activation function applied to the output.
• dropout (default 0): dropout rate
• reduce_output (default sum): defines how to reduce the output tensor along the s sequence length dimension if the rank of the tensor is greater than 2. Available values are: sum, mean or avg, max, concat (concatenates along the first dimension), last (returns the last vector of the first dimension) and null (which does not reduce and returns the full tensor).

Example sequence feature entry in the input features list using a parallel cnn encoder:

name: sequence_column_name
type: sequence
encoder: 
    type: stacked_parallel_cnn
    representation: dense
    embedding_size: 256
    embeddings_trainable: true
    filter_size: 3
    num_filters: 256
    pool_function: max
    output_size: 256
    use_bias: true
    weights_initializer: glorot_uniform
    bias_initializer: zeros
    activation: relu
    dropout: 0
    reduce_output: max

RNN Encoder

The rnn encoder works by first mapping the input integer sequence b x s (where b is the batch size and s is the length of the sequence) into a sequence of embeddings, then it passes the embedding through a stack of recurrent layers (by default 1 layer), followed by a reduce operation that by default only returns the last output, but can perform other reduce functions. If you want to output the full b x s x h where h is the size of the output of the last rnn layer, you can specify reduce_output: null.

       +------+
       |Emb 12|
       +------+
+--+   |Emb 7 |
|12|   +------+
|7 |   |Emb 43|                 +---------+
|43|   +------+   +----------+  |Fully    |
|65+--->Emb 65+--->RNN Layers+-->Connected+-->
|23|   +------+   +----------+  |Layers   |
|4 |   |Emb 23|                 +---------+
|1 |   +------+
+--+   |Emb 4 |
       +------+
       |Emb 1 |
       +------+

These are the available parameters for the rnn encoder:

• representation (default dense): the possible values are dense and sparse. dense means the embeddings are initialized randomly, sparse means they are initialized to be one-hot encodings.
• embedding_size (default 256): the maximum embedding size, the actual size will be min(vocabulary_size, embedding_size) for dense representations and exactly vocabulary_size for the sparse encoding, where vocabulary_size is the number of unique strings appearing in the training set input column plus the number of special tokens (<UNK>, <PAD>, <SOS>, <EOS>).
• embeddings_trainable (default true): If true embeddings are trained during the training process, if false embeddings are fixed. It may be useful when loading pretrained embeddings for avoiding finetuning them. This parameter has effect only when representation is dense as sparse one-hot encodings are not trainable.
• pretrained_embeddings (default null): by default dense embeddings are initialized randomly, but this parameter allows to specify a path to a file containing embeddings in the GloVe format. When the file containing the embeddings is loaded, only the embeddings with labels present in the vocabulary are kept, the others are discarded. If the vocabulary contains strings that have no match in the embeddings file, their embeddings are initialized with the average of all other embedding plus some random noise to make them different from each other. This parameter has effect only if representation is dense.
• embeddings_on_cpu (default false): by default embedding matrices are stored on GPU memory if a GPU is used, as it allows for faster access, but in some cases the embedding matrix may be too large. This parameter forces the placement of the embedding matrix in regular memory and the CPU is used for embedding lookup, slightly slowing down the process as a result of data transfer between CPU and GPU memory.
• num_layers (default 1): the number of stacked recurrent layers.
• state_size (default 256): the size of the state of the rnn.
• cell_type (default rnn): the type of recurrent cell to use. Available values are: rnn, lstm, gru. For reference about the differences between the cells please refer to torch.nn Recurrent Layers.
• bidirectional (default false): if true two recurrent networks will perform encoding in the forward and backward direction and their outputs will be concatenated.
• activation (default tanh): activation function to use.
• recurrent_activation (default sigmoid): activation function to use in the recurrent step
• unit_forget_bias (default true): If true, add 1 to the bias of the forget gate at initialization
• recurrent_initializer (default orthogonal): initializer for recurrent matrix weights
• dropout (default 0.0): dropout rate
• recurrent_dropout (default 0.0): dropout rate for recurrent state
• fc_layers (default null): a list of dictionaries containing the parameters of all the fully connected layers. The length of the list determines the number of stacked fully connected layers and the content of each dictionary determines the parameters for a specific layer. The available parameters for each layer are: activation, dropout, norm, norm_params, output_size, use_bias, bias_initializer and weights_initializer. If any of those values is missing from the dictionary, the default one specified as a parameter of the encoder will be used instead. If both fc_layers and num_fc_layers are null, a default list will be assigned to fc_layers with the value [{output_size: 512}, {output_size: 256}] (only applies if reduce_output is not null).
• num_fc_layers (default null): if fc_layers is null, this is the number of stacked fully connected layers (only applies if reduce_output is not null).
• output_size (default 256): if an output_size is not already specified in fc_layers this is the default output_size that will be used for each layer. It indicates the size of the output of a fully connected layer.
• use_bias (default true): boolean, whether the layer uses a bias vector.
• weights_initializer (default glorot_uniform): initializer for the weight matrix. Options are: constant, identity, zeros, ones, orthogonal, normal, uniform, truncated_normal, variance_scaling, glorot_normal, glorot_uniform, xavier_normal, xavier_uniform, he_normal, he_uniform, lecun_normal, lecun_uniform. Alternatively it is possible to specify a dictionary with a key type that identifies the type of initializer and other keys for its parameters, e.g. {type: normal, mean: 0, stddev: 0}. To know the parameters of each initializer, please refer to torch.nn.init.
• bias_initializer (default zeros): initializer for the bias vector. Options are: constant, identity, zeros, ones, orthogonal, normal, uniform, truncated_normal, variance_scaling, glorot_normal, glorot_uniform, xavier_normal, xavier_uniform, he_normal, he_uniform, lecun_normal, lecun_uniform. Alternatively it is possible to specify a dictionary with a key type that identifies the type of initializer and other keys for its parameters, e.g. {type: normal, mean: 0, stddev: 0}. To know the parameters of each initializer, please refer to torch.nn.init.
• norm (default null): if a norm is not already specified in fc_layers this is the default norm that will be used for each layer. It indicates how the output should be normalized and may be one of null, batch or layer.
• norm_params (default null): parameters used if norm is either batch or layer. For information on parameters used with batch see the Torch documentation on batch normalization or for layer see the Torch documentation on layer normalization.
• fc_activation (default relu): if an activation is not already specified in fc_layers this is the default activation that will be used for each layer. It indicates the activation function applied to the output.
• fc_dropout (default 0): dropout rate
• reduce_output (default last): defines how to reduce the output tensor along the s sequence length dimension if the rank of the tensor is greater than 2. Available values are: sum, mean or avg, max, concat (concatenates along the sequence dimension), last (returns the last vector of the sequence dimension) and null (which does not reduce and returns the full tensor).

Example sequence feature entry in the input features list using a parallel cnn encoder:

name: sequence_column_name
type: sequence
encoder: 
    type: rnn
    representation': dense
    embedding_size: 256
    embeddings_trainable: true
    num_layers: 1
    state_size: 256
    cell_type: rnn
    bidirectional: false
    activation: tanh
    recurrent_activation: sigmoid
    unit_forget_bias: true
    recurrent_initializer: orthogonal
    dropout: 0.0
    recurrent_dropout: 0.0
    output_size: 256
    use_bias: true
    weights_initializer: glorot_uniform
    bias_initializer: zeros
    fc_activation: relu
    fc_dropout: 0
    reduce_output: last

CNN RNN Encoder

The cnnrnn encoder works by first mapping the input integer sequence b x s (where b is the batch size and s is the length of the sequence) into a sequence of embeddings, then it passes the embedding through a stack of convolutional layers (by default 2), that is followed by a stack of recurrent layers (by default 1), followed by a reduce operation that by default only returns the last output, but can perform other reduce functions. If you want to output the full b x s x h where h is the size of the output of the last rnn layer, you can specify reduce_output: null.

       +------+
       |Emb 12|
       +------+
+--+   |Emb 7 |
|12|   +------+
|7 |   |Emb 43|                              +---------+
|43|   +------+  +----------+  +----------+  |Fully    |
|65+--->Emb 65+-->CNN Layers+-->RNN Layers+-->Connected+-->
|23|   +------+  +----------+  +----------+  |Layers   |
|4 |   |Emb 23|                              +---------+
|1 |   +------+
+--+   |Emb 4 |
       +------+
       |Emb 1 |
       +------+

These are the available parameters of the cnn rnn encoder:

• representation (default dense): the possible values are dense and sparse. dense means the embeddings are initialized randomly, sparse means they are initialized to be one-hot encodings.
• embedding_size (default 256): the maximum embedding size, the actual size will be min(vocabulary_size, embedding_size) for dense representations and exactly vocabulary_size for the sparse encoding, where vocabulary_size is the number of unique strings appearing in the training set input column plus the number of special tokens (<UNK>, <PAD>, <SOS>, <EOS>).
• embeddings_trainable (default true): If true embeddings are trained during the training process, if false embeddings are fixed. It may be useful when loading pretrained embeddings for avoiding finetuning them. This parameter has effect only when representation is dense as sparse one-hot encodings are not trainable.
• pretrained_embeddings (default null): by default dense embeddings are initialized randomly, but this parameter allows to specify a path to a file containing embeddings in the GloVe format. When the file containing the embeddings is loaded, only the embeddings with labels present in the vocabulary are kept, the others are discarded. If the vocabulary contains strings that have no match in the embeddings file, their embeddings are initialized with the average of all other embedding plus some random noise to make them different from each other. This parameter has effect only if representation is dense.
• embeddings_on_cpu (default false): by default embedding matrices are stored on GPU memory if a GPU is used, as it allows for faster access, but in some cases the embedding matrix may be too large. This parameter forces the placement of the embedding matrix in regular memory and the CPU is used for embedding lookup, slightly slowing down the process as a result of data transfer between CPU and GPU memory.
• conv_layers (default null): a list of dictionaries containing the parameters of all the convolutional layers. The length of the list determines the number of stacked convolutional layers and the content of each dictionary determines the parameters for a specific layer. The available parameters for each layer are: activation, dropout, norm, norm_params, num_filters, filter_size, strides, padding, dilation_rate, use_bias, pool_function, pool_padding, pool_size, pool_strides, bias_initializer, weights_initializer. If any of those values is missing from the dictionary, the default one specified as a parameter of the encoder will be used instead. If both conv_layers and num_conv_layers are null, a default list will be assigned to conv_layers with the value [{filter_size: 7, pool_size: 3}, {filter_size: 7, pool_size: 3}, {filter_size: 3, pool_size: null}, {filter_size: 3, pool_size: null}, {filter_size: 3, pool_size: null}, {filter_size: 3, pool_size: 3}].
• num_conv_layers (default 1): the number of stacked convolutional layers.
• num_filters (default 256): if a num_filters is not already specified in conv_layers this is the default num_filters that will be used for each layer. It indicates the number of filters, and by consequence the output channels of the 1d convolution.
• filter_size (default 5): if a filter_size is not already specified in conv_layers this is the default filter_size that will be used for each layer. It indicates how wide is the 1d convolutional filter.
• strides (default 1): stride length of the convolution
• padding (default same): one of valid or same.
• dilation_rate (default 1): dilation rate to use for dilated convolution
• conv_activation (default relu): activation for the convolution layer
• conv_dropout (default 0.0): dropout rate for the convolution layer
• pool_function (default max): pooling function: max will select the maximum value. Any of average, avg or mean will compute the mean value.
• pool_size (default 2 ): if a pool_size is not already specified in conv_layers this is the default pool_size that will be used for each layer. It indicates the size of the max pooling that will be performed along the s sequence dimension after the convolution operation.
• pool_strides (default null): factor to scale down
• pool_padding (default same): one of valid or same
• num_rec_layers (default 1): the number of recurrent layers
• state_size (default 256): the size of the state of the rnn.
• cell_type (default rnn): the type of recurrent cell to use. Available values are: rnn, lstm, gru. For reference about the differences between the cells please refer to torch.nn Recurrent Layers.
• bidirectional (default false): if true two recurrent networks will perform encoding in the forward and backward direction and their outputs will be concatenated.
• activation (default tanh): activation function to use
• recurrent_activation (default sigmoid): activation function to use in the recurrent step
• unit_forget_bias (default true): If true, add 1 to the bias of the forget gate at initialization
• recurrent_initializer (default orthogonal): initializer for recurrent matrix weights
• dropout (default 0.0): dropout rate
• recurrent_dropout (default 0.0): dropout rate for recurrent state
• fc_layers (default null): a list of dictionaries containing the parameters of all the fully connected layers. The length of the list determines the number of stacked fully connected layers and the content of each dictionary determines the parameters for a specific layer. The available parameters for each layer are: activation, dropout, norm, norm_params, output_size, use_bias, bias_initializer and weights_initializer. If any of those values is missing from the dictionary, the default one specified as a parameter of the encoder will be used instead. If both fc_layers and num_fc_layers are null, a default list will be assigned to fc_layers with the value [{output_size: 512}, {output_size: 256}] (only applies if reduce_output is not null).
• num_fc_layers (default null): if fc_layers is null, this is the number of stacked fully connected layers (only applies if reduce_output is not null).
• output_size (default 256): if an output_size is not already specified in fc_layers this is the default output_size that will be used for each layer. It indicates the size of the output of a fully connected layer.
• use_bias (default true): boolean, whether the layer uses a bias vector.
• weights_initializer (default glorot_uniform): initializer for the weights matrix. Options are: constant, identity, zeros, ones, orthogonal, normal, uniform, truncated_normal, variance_scaling, glorot_normal, glorot_uniform, xavier_normal, xavier_uniform, he_normal, he_uniform, lecun_normal, lecun_uniform. Alternatively it is possible to specify a dictionary with a key type that identifies the type of initializer and other keys for its parameters, e.g. {type: normal, mean: 0, stddev: 0}. To know the parameters of each initializer, please refer to torch.nn.init.
• bias_initializer (default zeros): initializer for the bias vector. Options are: constant, identity, zeros, ones, orthogonal, normal, uniform, truncated_normal, variance_scaling, glorot_normal, glorot_uniform, xavier_normal, xavier_uniform, he_normal, he_uniform, lecun_normal, lecun_uniform. Alternatively it is possible to specify a dictionary with a key type that identifies the type of initializer and other keys for its parameters, e.g. {type: normal, mean: 0, stddev: 0}. To know the parameters of each initializer, please refer to torch.nn.init.
• norm (default null): if a norm is not already specified in fc_layers this is the default norm that will be used for each layer. It indicates the norm of the output and it can be null, batch or layer.
• norm_params (default null): parameters used if norm is either batch or layer. For information on parameters used with batch see the Torch documentation on batch normalization or for layer see the Torch documentation on layer normalization.
• fc_activation (default relu): if an activation is not already specified in fc_layers this is the default activation that will be used for each layer. It indicates the activation function applied to the output.
• fc_dropout (default 0): dropout rate
• reduce_output (default last): defines how to reduce the output tensor along the s sequence length dimension if the rank of the tensor is greater than 2. Available values are: sum, mean or avg, max, concat (concatenates along the sequence dimension), last (returns the last vector of the sequence dimension) and null (which does not reduce and returns the full tensor).

Example sequence feature entry in the inputs features list using a cnn rnn encoder:

name: sequence_column_name
type: sequence
encoder: 
    type: cnnrnn
    representation: dense
    embedding_size: 256
    embeddings_trainable: true
    num_conv_layers: 1
    num_filters: 256
    filter_size: 5
    strides: 1
    padding: same
    dilation_rate: 1
    conv_activation: relu
    conv_dropout: 0.0
    pool_function: max
    pool_size: 2
    pool_padding: same
    num_rec_layers: 1
    state_size: 256
    cell_type: rnn
    bidirectional: false
    activation: tanh
    recurrent_activation: sigmoid
    unit_forget_bias: true
    recurrent_initializer: orthogonal
    dropout: 0.0
    recurrent_dropout: 0.0
    output_size: 256
    use_bias: true
    weights_initializer: glorot_uniform
    bias_initializer: zeros
    fc_activation: relu
    fc_dropout: 0
    reduce_output: last

Transformer Encoder

The transformer encoder implements a stack of transformer blocks, replicating the architecture introduced in the Attention is all you need paper, and adds am optional stack of fully connected layers at the end.

       +------+
       |Emb 12|
       +------+
+--+   |Emb 7 |
|12|   +------+
|7 |   |Emb 43|   +-------------+   +---------+
|43|   +------+   |             |   |Fully    |
|65+---+Emb 65+---> Transformer +--->Connected+-->
|23|   +------+   | Blocks      |   |Layers   |
|4 |   |Emb 23|   +-------------+   +---------+
|1 |   +------+
+--+   |Emb 4 |
       +------+
       |Emb 1 |
       +------+

• representation (default dense): the possible values are dense and sparse. dense means the embeddings are initialized randomly, sparse means they are initialized to be one-hot encodings.
• embedding_size (default 256): the maximum embedding size, the actual size will be min(vocabulary_size, embedding_size) for dense representations and exactly vocabulary_size for the sparse encoding, where vocabulary_size is the number of unique strings appearing in the training set input column plus the number of special tokens (<UNK>, <PAD>, <SOS>, <EOS>).
• embeddings_trainable (default true): If true embeddings are trained during the training process, if false embeddings are fixed. It may be useful when loading pretrained embeddings for avoiding finetuning them. This parameter has effect only when representation is dense as sparse one-hot encodings are not trainable.
• pretrained_embeddings (default null): by default dense embeddings are initialized randomly, but this parameter allows to specify a path to a file containing embeddings in the GloVe format. When the file containing the embeddings is loaded, only the embeddings with labels present in the vocabulary are kept, the others are discarded. If the vocabulary contains strings that have no match in the embeddings file, their embeddings are initialized with the average of all other embedding plus some random noise to make them different from each other. This parameter has effect only if representation is dense.
• embeddings_on_cpu (default false): by default embedding matrices are stored on GPU memory if a GPU is used, as it allows for faster access, but in some cases the embedding matrix may be too large. This parameter forces the placement of the embedding matrix in regular memory and the CPU is used for embedding lookup, slightly slowing down the process as a result of data transfer between CPU and GPU memory.
• num_layers (default 1): number of transformer blocks.
• hidden_size (default 256): the size of the hidden representation within the transformer block. It is usually the same as the embedding_size, but if the two values are different, a projection layer will be added before the first transformer block.
• num_heads (default 8): number of attention heads in each transformer block.
• transformer_output_size (default 256): Size of the fully connected layer after self attention in the transformer block. This is usually the same as hidden_size and embedding_size.
• dropout (default 0.1): dropout rate for the transformer block
• fc_layers (default null): a list of dictionaries containing the parameters of all the fully connected layers. The length of the list determines the number of stacked fully connected layers and the content of each dictionary determines the parameters for a specific layer. The available parameters for each layer are: activation, dropout, norm, norm_params, output_size, use_bias, bias_initializer and weights_initializer. If any of those values is missing from the dictionary, the default one specified as a parameter of the encoder will be used instead. If both fc_layers and num_fc_layers are null, a default list will be assigned to fc_layers with the value [{output_size: 512}, {output_size: 256}] (only applies if reduce_output is not null).
• num_fc_layers (default 0): This is the number of stacked fully connected layers (only applies if reduce_output is not null).
• output_size (default 256): if an output_size is not already specified in fc_layers this is the default output_size that will be used for each layer. It indicates the size of the output of a fully connected layer.
• use_bias (default true): boolean, whether the layer uses a bias vector.
• weights_initializer (default glorot_uniform): initializer for the weights matrix. Options are: constant, identity, zeros, ones, orthogonal, normal, uniform, truncated_normal, variance_scaling, glorot_normal, glorot_uniform, xavier_normal, xavier_uniform, he_normal, he_uniform, lecun_normal, lecun_uniform. Alternatively it is possible to specify a dictionary with a key type that identifies the type of initializer and other keys for its parameters, e.g. {type: normal, mean: 0, stddev: 0}. To know the parameters of each initializer, please refer to torch.nn.init.
• bias_initializer (default zeros): initializer for the bias vector. Options are: constant, identity, zeros, ones, orthogonal, normal, uniform, truncated_normal, variance_scaling, glorot_normal, glorot_uniform, xavier_normal, xavier_uniform, he_normal, he_uniform, lecun_normal, lecun_uniform. Alternatively it is possible to specify a dictionary with a key type that identifies the type of initializer and other keys for its parameters, e.g. {type: normal, mean: 0, stddev: 0}. To know the parameters of each initializer, please refer to torch.nn.init.
• norm (default null): if a norm is not already specified in fc_layers this is the default norm that will be used for each layer. It indicates the norm of the output and it can be null, batch or layer.
• norm_params (default null): parameters used if norm is either batch or layer. For information on parameters used with batch see the Torch documentation on batch normalization or for layer see the Torch documentation on layer normalization.
• fc_activation (default relu): if an activation is not already specified in fc_layers this is the default activation that will be used for each layer. It indicates the activation function applied to the output.
• fc_dropout (default 0): dropout rate
• reduce_output (default last): defines how to reduce the output tensor along the s sequence length dimension if the rank of the tensor is greater than 2. Available values are: sum, mean or avg, max, concat (concatenates along the sequence dimension), last (returns the last vector of the sequence dimension) and null (which does not reduce and returns the full tensor).

Example sequence feature entry in the inputs features list using a Transformer encoder:

name: sequence_column_name
type: sequence
encoder: 
    type: transformer
    representation: dense
    embedding_size: 256
    embeddings_trainable: true
    num_layers: 1
    hidden_size: 256
    num_heads: 8
    transformer_output_size: 256
    dropout: 0.1
    num_fc_layers: 0
    output_size: 256
    use_bias: true
    weights_initializer: glorot_uniform
    bias_initializer: zeros
    fc_activation: relu
    fc_dropout: 0
    reduce_output: last

Passthrough Encoder

The passthrough decoder simply transforms each input value into a float value and adds a dimension to the input tensor, creating a b x s x 1 tensor where b is the batch size and s is the length of the sequence. The tensor is reduced along the s dimension to obtain a single vector of size h for each element of the batch. If you want to output the full b x s x h tensor, you can specify reduce_output: null. This encoder is not really useful for sequence or text features, but may be useful for timeseries features, as it allows for using them without any processing in later stages of the model, like in a sequence combiner for instance.

+--+
|12|
|7 |                    +-----------+
|43|   +------------+   |Aggregation|
|65+--->Cast float32+--->Reduce     +-->
|23|   +------------+   |Operation  |
|4 |                    +-----------+
|1 |
+--+

These are the parameters available for the passthrough encoder

• reduce_output (default null): defines how to reduce the output tensor along the s sequence length dimension if the rank of the tensor is greater than 2. Available values are: sum, mean or avg, max, concat (concatenates along the sequence dimension), last (returns the last vector of the sequence dimension) and null (which does not reduce and returns the full tensor).

Example sequence feature entry in the input features list using a passthrough encoder:

name: sequence_column_name
type: sequence
encoder: 
    type: passthrough
    reduce_output: null

Sequence Output Features and Decoders

Sequence output features can be used for either tagging (classifying each element of an input sequence) or generation (generating a sequence by sampling from the model). Ludwig provides two sequence decoders named tagger and generator respectively.

The following are the available parameters of a sequence output feature:

• reduce_input (default sum): defines how to reduce an input that is not a vector, but a matrix or a higher order tensor, on the first dimension (second if you count the batch dimension). Available values are: sum, mean or avg, max, concat (concatenates along the sequence dimension), last (returns the last vector of the sequence dimension).
• dependencies (default []): the output features this one is dependent on. For a detailed explanation refer to Output Feature Dependencies.
• reduce_dependencies (default sum): defines how to reduce the output of a dependent feature that is not a vector, but a matrix or a higher order tensor, on the first dimension (second if you count the batch dimension). Available values are: sum, mean or avg, max, concat (concatenates along the sequence dimension), last (returns the last vector of the sequence dimension).
• loss (default {type: softmax_cross_entropy, class_similarities_temperature: 0, class_weights: 1, confidence_penalty: 0, robust_lambda: 0}): is a dictionary containing a loss type. The only available loss type for sequences is softmax_cross_entropy. For more details on losses and their options, see also Category Output Features and Decoders.

Decoder type and decoder parameters can also be defined once and applied to all sequence output features using the Type-Global Decoder section. Loss and loss related parameters can also be defined once in the same way.

Tagger Decoder

In the case of tagger the decoder is a (potentially empty) stack of fully connected layers, followed by a projection into a tensor of size b x s x c, where b is the batch size, s is the length of the sequence and c is the number of classes, followed by a softmax_cross_entropy. This decoder requires its input to be shaped as b x s x h, where h is a hidden dimension, which is the output of a sequence, text or time series input feature without reduced outputs or the output of a sequence-based combiner. If a b x h input is provided instead, an error will be raised during model building.

Combiner
Output

+---+                 +----------+   +-------+
|emb|   +---------+   |Projection|   |Softmax|
+---+   |Fully    |   +----------+   +-------+
|...+--->Connected+--->...       +--->...    |
+---+   |Layers   |   +----------+   +-------+
|emb|   +---------+   |Projection|   |Softmax|
+---+                 +----------+   +-------+

These are the available parameters of a tagger decoder:

• fc_layers (default null): a list of dictionaries containing the parameters of all the fully connected layers. The length of the list determines the number of stacked fully connected layers and the content of each dictionary determines the parameters for a specific layer. The available parameters for each layer are: activation, dropout, norm, norm_params, output_size, use_bias, bias_initializer and weights_initializer. If any of those values is missing from the dictionary, the default one specified as a parameter of the decoder will be used instead.
• num_fc_layers (default 0): the number of stacked fully connected layers that the input to the feature passes through. Their output is projected in the feature's output space.
• output_size (default 256): if an output_size is not already specified in fc_layers this is the default output_size that will be used for each layer. It indicates the size of the output of a fully connected layer.
• use_bias (default true): boolean, whether the layer uses a bias vector.
• weights_initializer (default glorot_uniform): initializer for the weights matrix. Options are: constant, identity, zeros, ones, orthogonal, normal, uniform, truncated_normal, variance_scaling, glorot_normal, glorot_uniform, xavier_normal, xavier_uniform, he_normal, he_uniform, lecun_normal, lecun_uniform. Alternatively it is possible to specify a dictionary with a key type that identifies the type of initializer and other keys for its parameters, e.g. {type: normal, mean: 0, stddev: 0}. To know the parameters of each initializer, please refer to torch.nn.init.
• bias_initializer (default zeros): initializer for the bias vector. Options are: constant, identity, zeros, ones, orthogonal, normal, uniform, truncated_normal, variance_scaling, glorot_normal, glorot_uniform, xavier_normal, xavier_uniform, he_normal, he_uniform, lecun_normal, lecun_uniform. Alternatively it is possible to specify a dictionary with a key type that identifies the type of initializer and other keys for its parameters, e.g. {type: normal, mean: 0, stddev: 0}. To know the parameters of each initializer, please refer to torch.nn.init.
• norm (default null): if a norm is not already specified in fc_layers this is the default norm that will be used for each layer. It indicates how the output should be normalized and may be one of null, batch or layer.
• norm_params (default null): parameters used if norm is either batch or layer. For information on parameters used with batch see the Torch documentation on batch normalization or for layer see the Torch documentation on layer normalization.
• activation (default relu): if an activation is not already specified in fc_layers this is the default activation that will be used for each layer. It indicates the activation function applied to the output.
• dropout (default 0): dropout rate
• attention (default false): If true, applies a multi-head self attention layer before prediction.
• attention_embedding_size (default 256): the embedding size of the multi-head self attention layer.
• attention_num_heads (default 8): number of attention heads in the multi-head self attention layer.

Example sequence feature entry using a tagger decoder (with default parameters) in the output features list:

name: sequence_column_name
type: sequence
reduce_input: null
dependencies: []
reduce_dependencies: sum
loss:
    type: softmax_cross_entropy
    confidence_penalty: 0
    robust_lambda: 0
    class_weights: 1
    class_similarities_temperature: 0
decoder: 
    type: tagger
    num_fc_layers: 0
    output_size: 256
    use_bias: true
    weights_initializer: glorot_uniform
    bias_initializer: zeros
    activation: relu
    dropout: 0
    attention: false
    attention_embedding_size: 256
    attention_num_heads: 8

Generator Decoder

In the case of generator the decoder is a (potentially empty) stack of fully connected layers, followed by an rnn that generates outputs feeding on its own previous predictions and generates a tensor of size b x s' x c, where b is the batch size, s' is the length of the generated sequence and c is the number of classes, followed by a softmax_cross_entropy. During training teacher forcing is adopted, meaning the list of targets is provided as both inputs and outputs (shifted by 1), while at evaluation time greedy decoding (generating one token at a time and feeding it as input for the next step) is performed by beam search, using a beam of 1 by default. In general a generator expects a b x h shaped input tensor, where h is a hidden dimension. The h vectors are (after an optional stack of fully connected layers) fed into the rnn generator. One exception is when the generator uses attention, as in that case the expected size of the input tensor is b x s x h, which is the output of a sequence, text or time series input feature without reduced outputs or the output of a sequence-based combiner. If a b x h input is provided to a generator decoder using an rnn with attention instead, an error will be raised during model building.

                            Output     Output
                               1  +-+    ... +--+    END
                               ^    |     ^     |     ^
+--------+   +---------+       |    |     |     |     |
|Combiner|   |Fully    |   +---+--+ | +---+---+ | +---+--+
|Output  +--->Connected+---+RNN   +--->RNN... +--->RNN   |
|        |   |Layers   |   +---^--+ | +---^---+ | +---^--+
+--------+   +---------+       |    |     |     |     |
                              GO    +-----+     +-----+

• reduce_input (default sum): defines how to reduce an input that is not a vector, but a matrix or a higher order tensor, on the first dimension (second if you count the batch dimension). Available values are: sum, mean or avg, max, concat (concatenates along the sequence dimension), last (returns the last vector of the sequence dimension).

These are the available parameters of a Generator decoder:

• fc_layers (default null): it is a list of dictionaries containing the parameters of all the fully connected layers. The length of the list determines the number of stacked fully connected layers and the content of each dictionary determines the parameters for a specific layer. The available parameters for each layer are: activation, dropout, norm, norm_params, output_size, use_bias, bias_initializer and weights_initializer. If any of those values is missing from the dictionary, the default one specified as a parameter of the decoder will be used instead.
• num_fc_layers (default 0): this is the number of stacked fully connected layers that the input to the feature passes through. Their output is projected in the feature's output space.
• output_size (default 256): if an output_size is not already specified in fc_layers this is the default output_size that will be used for each layer. It indicates the size of the output of a fully connected layer.
• use_bias (default true): boolean, whether the layer uses a bias vector.
• weights_initializer (default glorot_uniform): initializer for the weight matrix. Options are: constant, identity, zeros, ones, orthogonal, normal, uniform, truncated_normal, variance_scaling, glorot_normal, glorot_uniform, xavier_normal, xavier_uniform, he_normal, he_uniform, lecun_normal, lecun_uniform. Alternatively it is possible to specify a dictionary with a key type that identifies the type of initializer and other keys for its parameters, e.g. {type: normal, mean: 0, stddev: 0}. To know the parameters of each initializer, please refer to torch.nn.init.
• bias_initializer (default zeros): initializer for the bias vector. Options are: constant, identity, zeros, ones, orthogonal, normal, uniform, truncated_normal, variance_scaling, glorot_normal, glorot_uniform, xavier_normal, xavier_uniform, he_normal, he_uniform, lecun_normal, lecun_uniform. Alternatively it is possible to specify a dictionary with a key type that identifies the type of initializer and other keys for its parameters, e.g. {type: normal, mean: 0, stddev: 0}. To know the parameters of each initializer, please refer to torch.nn.init.
• norm (default null): if a norm is not already specified in fc_layers this is the default norm that will be used for each layer. It indicates how the output should be normalized and may be one of null, batch or layer.
• norm_params (default null): parameters used if norm is either batch or layer. For information on parameters used with batch see Torch documentation on batch normalization or for layer see Torch documentation on layer normalization.
• activation (default relu): if an activation is not already specified in fc_layers this is the default activation that will be used for each layer. It indicates the activation function applied to the output.
• dropout (default 0): dropout rate
• cell_type (default rnn): the type of recurrent cell to use. Available values are: rnn, lstm, gru. For reference about the differences between the cells please refer to torch.nn Recurrent Layers.
• state_size (default 256): the size of the state of the rnn.
• embedding_size (default 256): The size of the embeddings of the inputs of the generator.
• beam_width (default 1): sampling from the RNN generator is performed using beam search. By default, with a beam of one, only a greedy sequence using always the most probable next token is generated, but the beam size can be increased. This usually leads to better performance at the expense of more computation and slower generation.
• tied (default null): if null the embeddings of the targets are initialized randomly. If tied names an input feature, the embeddings of that input feature will be used as embeddings of the target. The vocabulary_size of that input feature has to be the same as the output feature and it has to have an embedding matrix (binary and number features will not have one, for instance). In this case the embedding_size will be the same as the state_size. This is useful for implementing autoencoders where the encoding and decoding part of the model share parameters.
• max_sequence_length (default 256): The maximum sequence length.

Example sequence feature entry using a generator decoder in the output features list:

name: sequence_column_name
type: sequence
reduce_input: sum
dependencies: []
reduce_dependencies: sum
loss:
    type: softmax_cross_entropy
    confidence_penalty: 0
    robust_lambda: 0
    class_weights: 1
    class_similarities_temperature: 0
decoder: 
    type: generator
    num_fc_layers: 0
    output_size: 256
    use_bias: true
    bias_initializer: zeros
    weights_initializer: glorot_uniform
    activation: relu
    dropout: 0
    cell_type: rnn
    state_size: 256
    embedding_size: 256
    beam_width: 1
    max_sequence_length: 256

Sequence Features Metrics

The metrics that are calculated every epoch and are available for sequence features are:

• sequence_accuracy The rate at which the model predicted the correct sequence.
• token_accuracy The number of tokens correctly predicted divided by the total number of tokens in all sequences.
• last_accuracy Accuracy considering only the last element of the sequence. Useful to ensure special end-of-sequence tokens are generated or tagged.
• edit_distance Levenshtein distance: the minimum number of single-token edits (insertions, deletions or substitutions) required to change predicted sequence to ground truth.
• perplexity Perplexity is the inverse of the predicted probability of the ground truth sequence, normalized by the number of tokens. The lower the perplexity, the higher the probability of predicting the true sequence.
• loss The value of the loss function.

You can set any of the above as validation_metric in the training section of the configuration if validation_field names a sequence feature.