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Add a Combiner

Combiners are responsible for combining the outputs of one or more input features into a single combined representation, which is usually a vector, but may also be a sequence of vectors or some other higher-dimensional tensor. One or more output features will use this combined representation to generate predictions.

Users can specify which combiner to use in the combiner section of the configuration; if a combiner is not specified the concat combiner will be used.

Recall the ECD (Encoder, Combiner, Decoder) data flow architecture: all input feature outputs flow into the combiner, and the combiner's output flows into all output features.

+-----------+                      +-----------+
|Input      |                      | Output    |
|Feature 1  +-+                  +-+ Feature 1 + ---> Prediction 1
+-----------+ |                  | +-----------+
+-----------+ |   +----------+   | +-----------+
|...        +---> | Combiner +---> |...        +
+-----------+ |   +----------+   | +-----------+
+-----------+ |                  | +-----------+
|Input      +-+                  +-+ Output    |
|Feature N  |                      | Feature N + ---> Prediction N
+-----------+                      +-----------+

There is an additional complication to keep in mind: input features may either output vectors, or sequences of vectors. Thus, a combiner may have to handle a mix of input features whose outputs are of different dimensionality. SequenceConcatCombiner, for example, resolves this by requiring that all input sequences be of the same length. It will raise a ValueError exception if they are not. SequenceConcatCombiner tiles non-sequence inputs to the sequence length before concatenation, processing all input features as sequences of the same length.

New combiners should make it clear in their doc strings if they support sequence inputs, declare any requirements on sequence length, type, or dimension, and validate their input features.

In this guide we'll outline how to extend Ludwig by adding a new combiner, using the transformer combiner as a template. To add a new combiner:

  1. Define a dataclass to represent the combiner schema.
  2. Create a new combiner class inheriting from ludwig.combiners.Combiner or one of its subclasses.
  3. Allocate all layers and state in the __init__ method.
  4. Implement your combiner's forward pass in def forward(self, inputs: Dict):.
  5. Add tests.
  6. Add the new combiner to the combiner registry.

1. Define the combiner's schema

The combiner schema is a dataclass (overloaded by the marshmallow_dataclass modue) that must extend BaseCombinerConfig. Its attributes are the configuration parameters of the combiner. All fields should have a type and a default value. The module provides convenient methods for specifying the valid types and ranges of a combiner config. For example, the TransformerCombiner has the following schema:

from typing import Optional, List, Dict, Any

# Main imports:
from marshmallow_dataclass import dataclass
from ludwig.schema import utils as schema_utils
from ludwig.schema.combiners.base import BaseCombinerConfig

class TransformerCombinerConfig(BaseCombinerConfig):
    num_layers: int = schema.PositiveInteger(default=1)
    hidden_size: int = schema.NonNegativeInteger(default=256)
    num_heads: int = schema.NonNegativeInteger(default=8)
    transformer_output_size: int = schema.NonNegativeInteger(default=256)
    dropout: float = schema.FloatRange(default=0.1, min=0, max=1)
    fc_layers: Optional[List[Dict[str, Any]]] = schema.DictList()
    num_fc_layers: int = schema.NonNegativeInteger(default=0)
    output_size: int = schema.PositiveInteger(default=256)
    use_bias: bool = True
    weights_initializer: Union[str, Dict] = \
    bias_initializer: Union[str, Dict] = \
    norm: Optional[str] = schema.StringOptions(["batch", "layer"])
    norm_params: Optional[dict] = schema.Dict()
    fc_activation: str = "relu"
    fc_dropout: float = schema.FloatRange(default=0.0, min=0, max=1)
    fc_residual: bool = False
    reduce_output: Optional[str] = schema.ReductionOptions(default="mean")

This schema should live in its own file inside ludwig/schema/combiners/. So that it is more convenient to import elsewhere in Ludwig, you may also add it as an import to ludwig/schema/combiners/ like so:

from ludwig.schema.combiners.transformer import TransformerCombinerConfig  # noqa: F401

2. Add a new combiner class

Source code for combiners lives in ludwig/combiners/. Add a new python module which declares a new combiner class. For this example, we'll show how to implement a simplified version of transformer combiner which would be defined in


At present, all combiners are defined in ludwig/combiners/ However, for new combiners we recommend creating a new python module with a name corresponding to the new combiner class.

class TransformerCombiner(Combiner):
    def __init__(
        input_features: Dict[str, InputFeature] = None,
        config: TransformerCombinerConfig = None,
        super().__init__(input_features) = "TransformerCombiner"

    def forward(
        inputs: Dict,
    ) -> Dict[str: torch.Tensor]:

    def get_schema_cls():
        return TransformerCombinerConfig

Implement @staticmethod def get_schema_cls(): and return the class name of your config schema.

3. Implement Constructor

The combiner constructor will be initialized with a dictionary of the input features and the combiner config. The constructor must pass the input features to the superclass constructor, set its name property, then create its own layers and state.

The input_features dictionary is passed in to the constructor to make information about the number, size, and type of the inputs accessible. This may determine what resources the combiner needs to allocate. For example, the transformer combiner treats its input features as a sequence, where the sequence length is the number of features. We can determine the sequence length here as self.sequence_size = len(self.input_features).

    def __init__(
        input_features: Dict[str, InputFeature] = None,
        config: TransformerCombinerConfig = None,
        super().__init__(input_features) = "TransformerCombiner"
        # ...
        self.sequence_size = len(self.input_features)

        self.transformer_stack = TransformerStack(
        # ...

4. Implement forward method

The forward method of the combiner should combine the input feature representations into a single output tensor, which will be passed to output feature decoders. Each key in inputs is an input feature name, and the respective value is a dictionary of the input feature's outputs. Each feature output dictionary is guaranteed to contain an encoder_output key, and may contain other outputs depending on the encoder.

forward returns a dictionary mapping strings to tensors which must contain a combiner_output key. It may optionally return additional values that might be useful for output feature decoding, loss computation, or explanation. For example, TabNetCombiner returns its sparse attention masks (attention_masks, and aggregated_attention_masks) which are useful to see which input features were attended to in each prediction step.

For example, the following is a simplified version of TransformerCombiner's forward method:

    def forward(
        self, inputs: Dict[str, Dict[str, torch.Tensor]]
    ) -> Dict[str, torch.Tensor]:
        encoder_outputs = [inputs[k]["encoder_output"] for k in inputs]

        # ================ Flatten ================
        batch_size = encoder_outputs[0].shape[0]
        encoder_outputs = [
            torch.reshape(eo, [batch_size, -1]) for eo in encoder_outputs

        # ================ Project & Concat ================
        projected = [
            self.projectors[i](eo) for i, eo in enumerate(encoder_outputs)
        hidden = torch.stack(projected)
        hidden = torch.permute(hidden, (1, 0, 2))

        # ================ Transformer Layers ================
        hidden = self.transformer_stack(hidden)

        # ================ Sequence Reduction ================
        if self.reduce_output is not None:
            hidden = self.reduce_sequence(hidden)
            hidden = self.fc_stack(hidden)

        return_data = {"combiner_output": hidden}
        return return_data


  • inputs (Dict[str, Dict[str, torch.Tensor]]): A dictionary of input feature outputs, keyed by the input feature names. Each input feature output dictionary is guaranteed to include encoder_output, and may include other key/value pairs depending on the input feature's encoder.


  • (Dict[str, torch.Tensor]): A dictionary containing the required key combiner_output whose value is the combiner output tensor, and any other optional output key/value pairs.

5. Add new class to the registry

Mapping between combiner names in the model config and combiner classes is made by registering the class in the combiner registry. The combiner registry is defined in ludwig/schema/combiners/ To register your class, add the @register_combiner decorator on the line above its class definition, specifying the name of the combiner:

class TransformerCombiner(Combiner):

6. Add tests

Add a corresponding unit test module to tests/ludwig/combiners, using the name of your combiner module prefixed by test_ i.e.

At a minimum, the unit test should ensure that:

  1. The combiner's forward pass succeeds for all feature types it supports.
  2. The combiner fails in expected ways when given unsupported input. (Skip this if the combiner supports all input feature types.)
  3. The combiner produces output of the correct type and dimensionality given a variety of configs.

Use @pytest.mark.parametrize to parameterize your test with different configurations, also test edge cases:

@pytest.mark.parametrize("output_size", [8, 16])
@pytest.mark.parametrize("transformer_output_size", [4, 12])
def test_transformer_combiner(
        encoder_outputs: tuple,
        transformer_output_size: int,
        output_size: int) -> None:
    encoder_outputs_dict, input_feature_dict = encoder_outputs

For examples of combiner tests, see tests/ludwig/combiners/

For more detail about unit testing in Ludiwg, see also Unit Test Design Guidelines.