Add a Decoder

1. Add a new decoder class

Source code for decoders lives under ludwig/decoders/. New decoder objects should be defined in the corresponding files, for example all new sequence decoders should be added to ludwig/decoders/sequence_decoders.py.

All the decoder parameters should be provided as arguments in the constructor with their default values set. For example the SequenceGeneratorDecoder decoder takes the following list of arguments in its constructor:

def __init__(
    self,
    num_classes,
    cell_type='rnn',
    state_size=256,
    embedding_size=64,
    beam_width=1,
    num_layers=1,
    attention=None,
    tied_embeddings=None,
    is_timeseries=False,
    max_sequence_length=0,
    use_bias=True,
    weights_initializer='glorot_uniform',
    bias_initializer='zeros',
    weights_regularizer=None,
    bias_regularizer=None,
    activity_regularizer=None,
    reduce_input='sum',
    **kwargs
):

Decoders are initialized as class member variables in output feature object constructors and called inside call methods.

2. Add the new decoder class to the corresponding decoder registry

Mapping between decoder names in the model definition and decoder classes in the codebase is done by encoder registries: for example sequence encoder registry is defined in ludwig/features/sequence_feature.py inside the SequenceOutputFeature as:

sequence_decoder_registry = {
    'generator': Generator,
    'tagger': Tagger
}

All you have to do to make you new decoder available as an option in the model definition is to add it to the appropriate registry.