Bi-directional Transformer-based encoder network.

This network implements a bi-directional Transformer-based encoder as described in "BERT: Pre-training of Deep Bidirectional Transformers for Language Understanding" ( It includes the embedding lookups and transformer layers, but not the masked language model or classification task networks.

The default values for this object are taken from the BERT-Base implementation in "BERT: Pre-training of Deep Bidirectional Transformers for Language Understanding".

vocab_size The size of the token vocabulary.
hidden_size The size of the transformer hidden layers.
num_layers The number of transformer layers.
num_attention_heads The number of attention heads for each transformer. The hidden size must be divisible by the number of attention heads.
max_sequence_length The maximum sequence length that this encoder can consume. This determines the variable shape for positional embeddings.
type_vocab_size The number of types that the 'type_ids' input can take.
inner_dim The output dimension of the first Dense layer in a two-layer feedforward network for each transformer.
inner_activation The activation for the first Dense layer in a two-layer feedforward network for each transformer.
output_dropout Dropout probability for the post-attention and output dropout.
attention_dropout The dropout rate to use for the attention layers within the transformer layers.
initializer The initialzer to use for all weights in this encoder.
output_range The sequence output range, [0, output_range), by slicing the target sequence of the last transformer layer. None means the entire target sequence will attend to the source sequence, which yields the full output.
embedding_width The width of the word embeddings. If the embedding width is not equal to hidden size, embedding parameters will be factorized into two matrices in the shape of ['vocab_size', 'embedding_width'] and 'embedding_width', 'hidden_size'.
embedding_layer An optional Layer instance which will be called to generate embeddings for the input word IDs.
norm_first Whether to normalize inputs to attention and intermediate dense layers. If set False, output of attention and intermediate dense layers is normalized.
with_dense_inputs Whether to accept dense embeddings as the input.
return_attention_scores Whether to add an additional output containing the attention scores of all transformer layers. This will be a list of length num_layers, and each element will be in the shape [batch_size, num_attention_heads, seq_dim, seq_dim].

pooler_layer The pooler dense layer after the transformer layers.
transformer_layers List of Transformer layers in the encoder.



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This is where the layer's logic lives.

The call() method may not create state (except in its first invocation, wrapping the creation of variables or other resources in tf.init_scope()). It is recommended to create state, including tf.Variable instances and nested Layer instances, in __init__(), or in the build() method that is called automatically before call() executes for the first time.

inputs Input tensor, or dict/list/tuple of input tensors. The first positional inputs argument is subject to special rules:

  • inputs must be explicitly passed. A layer cannot have zero arguments, and inputs cannot be provided via the default value of a keyword argument.
  • NumPy array or Python scalar values in inputs get cast as tensors.
  • Keras mask metadata is only collected from inputs.
  • Layers are built (build(input_shape) method) using shape info from inputs only.
  • input_spec compatibility is only checked against inputs.
  • Mixed precision input casting is only applied to inputs. If a layer has tensor arguments in *args or **kwargs, their casting behavior in mixed precision should be handled manually.
  • The SavedModel input specification is generated using inputs only.
  • Integration with various ecosystem packages like TFMOT, TFLite, TF.js, etc is only supported for inputs and not for tensors in positional and keyword arguments.
*args Additional positional arguments. May contain tensors, although this is not recommended, for the reasons above.
**kwargs Additional keyword arguments. May contain tensors, although this is not recommended, for the reasons above. The following optional keyword arguments are reserved:
  • training: Boolean scalar tensor of Python boolean indicating whether the call is meant for training or inference.
  • mask: Boolean input mask. If the layer's call() method takes a mask argument, its default value will be set to the mask generated for inputs by the previous layer (if input did come from a layer that generated a corresponding mask, i.e. if it came from a Keras layer with masking support).
  • Returns
    A tensor or list/tuple of tensors.


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