transformer_xl.py

# Copyright 2021 The TensorFlow Authors. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
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#     http://www.apache.org/licenses/LICENSE-2.0
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"""Keras-based Transformer XL layer."""

from absl import logging

import tensorflow as tf

from official.nlp.modeling.layers import relative_attention


def _cache_memory(current_state, previous_state, memory_length, reuse_length=0):
  """Caches hidden states into memory.

  Args:
    current_state: `Tensor`, the current state.
    previous_state: `Tensor`, the previous state.
    memory_length: `int`, the number of tokens to cache.
    reuse_length: `int`, the number of tokens in the current batch to be cached
      and reused in the future.

  Returns:
    A `Tensor`, representing the cached state with stopped gradients.

  """
  if memory_length is None or memory_length == 0:
    return None
  else:
    if reuse_length > 0:
      current_state = current_state[:, :reuse_length, :]

    if previous_state is None:
      new_mem = current_state[:, -memory_length:, :]
    else:
      new_mem = tf.concat(
          [previous_state, current_state], 1)[:, -memory_length:, :]

  return tf.stop_gradient(new_mem)


@tf.keras.utils.register_keras_serializable(package="Text")
class TransformerXLBlock(tf.keras.layers.Layer):
  """Transformer XL block.

  This implements a Transformer XL block from "Transformer-XL: Attentive
  Language Models Beyond a Fixed-Length Context"
  (https://arxiv.org/abs/1901.02860).

  This block is further extended to allow for the Transformer-XL
  re-parameterization in "XLNet: Generalized Autoregressive Pretraining for
  Language Understanding" (https://arxiv.org/abs/1906.08237).

  Given an input stream, this block computes attention, applies dropouts and
  layer norms and feeds into the FFN network.

  **Note: This layer is currently experimental.

  Attributes:
    vocab_size: The size of the token vocabulary.
    hidden_size: The size of the transformer hidden layers.
    num_attention_heads: The number of attention heads.
    head_size: The dimension size of each attention head.
    inner_size: The inner size for the transformer layers.
    dropout_rate: Dropout rate for the output of this layer.
    attention_dropout_rate: Dropout rate on attention probabilities.
    two_stream: Whether or not to use `TwoStreamRelativeAttention` used in the
      XLNet pretrainer. If `False`, then it will use
      `MultiHeadRelativeAttention` as in Transformer XL.
    norm_epsilon: Epsilon value to initialize normalization layers.
    inner_activation: The activation to use for the inner
      FFN layers.
    kernel_initializer: Initializer for dense layer kernels.
    inner_dropout: Dropout probability for the inner dropout
      layer.
  """

  def __init__(self,
               vocab_size,
               hidden_size,
               num_attention_heads,
               head_size,
               inner_size,
               dropout_rate,
               attention_dropout_rate,
               two_stream=False,
               norm_epsilon=1e-12,
               inner_activation="relu",
               kernel_initializer="variance_scaling",
               inner_dropout=0.0,
               **kwargs):
    """Initializes TransformerXLBlock layer."""

    super(TransformerXLBlock, self).__init__(**kwargs)
    self._vocab_size = vocab_size
    self._num_heads = num_attention_heads
    self._head_size = head_size
    self._hidden_size = hidden_size
    self._inner_size = inner_size
    self._dropout_rate = dropout_rate
    self._attention_dropout_rate = attention_dropout_rate
    self._inner_activation = inner_activation
    self._norm_epsilon = norm_epsilon
    self._kernel_initializer = kernel_initializer
    self._inner_dropout = inner_dropout
    self._two_stream = two_stream
    if two_stream:
      self._attention_layer_type = relative_attention.TwoStreamRelativeAttention
    else:
      self._attention_layer_type = relative_attention.MultiHeadRelativeAttention

  def build(self, input_shape):
    input_tensor = input_shape[0] if len(input_shape) == 2 else input_shape
    input_tensor_shape = tf.TensorShape(input_tensor)
    if len(input_tensor_shape.as_list()) != 3:
      raise ValueError("TransformerLayer expects a three-dimensional input of "
                       "shape [batch, sequence, width].")
    batch_size, sequence_length, hidden_size = input_tensor_shape

    if len(input_shape) == 2:
      mask_tensor_shape = tf.TensorShape(input_shape[1])
      expected_mask_tensor_shape = tf.TensorShape(
          [batch_size, sequence_length, sequence_length])
      if not expected_mask_tensor_shape.is_compatible_with(mask_tensor_shape):
        raise ValueError("When passing a mask tensor to TransformerXLBlock, "
                         "the mask tensor must be of shape [batch, "
                         "sequence_length, sequence_length] (here %s). Got a "
                         "mask tensor of shape %s." %
                         (expected_mask_tensor_shape, mask_tensor_shape))
    if hidden_size % self._num_heads != 0:
      raise ValueError(
          "The input size (%d) is not a multiple of the number of attention "
          "heads (%d)" % (hidden_size, self._num_heads))
    self._attention_layer = self._attention_layer_type(
        num_heads=self._num_heads,
        key_dim=self._head_size,
        value_dim=self._head_size,
        dropout=self._attention_dropout_rate,
        use_bias=False,
        kernel_initializer=self._kernel_initializer,
        name="rel_attn")
    self._attention_dropout = tf.keras.layers.Dropout(
        rate=self._attention_dropout_rate)
    self._attention_layer_norm = tf.keras.layers.LayerNormalization(
        name="self_attention_layer_norm",
        axis=-1,
        epsilon=self._norm_epsilon,
        dtype=tf.float32)
    self._inner_dense = tf.keras.layers.experimental.EinsumDense(
        "abc,cd->abd",
        output_shape=(None, self._inner_size),
        bias_axes="d",
        kernel_initializer=self._kernel_initializer,
        name="inner")

    self._inner_activation_layer = tf.keras.layers.Activation(
        self._inner_activation)
    self._inner_dropout_layer = tf.keras.layers.Dropout(
        rate=self._inner_dropout)
    self._output_dense = tf.keras.layers.experimental.EinsumDense(
        "abc,cd->abd",
        output_shape=(None, hidden_size),
        bias_axes="d",
        name="output",
        kernel_initializer=self._kernel_initializer)
    self._output_dropout = tf.keras.layers.Dropout(rate=self._dropout_rate)
    self._output_layer_norm = tf.keras.layers.LayerNormalization(
        name="output_layer_norm",
        axis=-1,
        epsilon=self._norm_epsilon)

    super(TransformerXLBlock, self).build(input_shape)

  def get_config(self):
    config = {
        "vocab_size":
            self._vocab_size,
        "hidden_size":
            self._hidden_size,
        "num_attention_heads":
            self._num_heads,
        "head_size":
            self._head_size,
        "inner_size":
            self._inner_size,
        "dropout_rate":
            self._dropout_rate,
        "attention_dropout_rate":
            self._attention_dropout_rate,
        "two_stream":
            self._two_stream,
        "norm_epsilon":
            self._norm_epsilon,
        "inner_activation":
            self._inner_activation,
        "kernel_initializer":
            self._kernel_initializer,
        "inner_dropout":
            self._inner_dropout,
    }
    base_config = super(TransformerXLBlock, self).get_config()
    return dict(list(base_config.items()) + list(config.items()))

  def call(self,
           content_stream,
           content_attention_bias,
           positional_attention_bias,
           relative_position_encoding=None,
           segment_matrix=None,
           segment_encoding=None,
           segment_attention_bias=None,
           state=None,
           content_attention_mask=None,
           query_stream=None,
           query_attention_mask=None,
           target_mapping=None):
    """Implements `call` for the Layer.

    Args:
      content_stream: `Tensor`, the input content stream. This is the standard
        input to Transformer XL and is commonly referred to as `h` in XLNet.
      content_attention_bias: Bias `Tensor` for content based attention of shape
        `[num_heads, dim]`.
      positional_attention_bias: Bias `Tensor` for position based attention of
        shape `[num_heads, dim]`.
      relative_position_encoding: Relative positional encoding `Tensor` of shape
        `[B, L, dim]`.
      segment_matrix: Optional `Tensor` of shape `[B, S, S + M]`. Used in XLNet,
        but not in Transformer XL.
      segment_encoding: Optional `Tensor` of shape `[2, num_heads, dim]`. Used
        in XLNet, but not in Transformer XL.
      segment_attention_bias: Optional bias `Tensor` for segment based attention
        of shape `[num_heads, dim]`.
      state: Optional `Tensor` of shape `[B, M, E]`, where M is the length of
        the state or memory. If passed, this is also attended over as in
        Transformer XL.
      content_attention_mask: Optional `Tensor` representing the mask that is
        added to content attention logits. If state is not None, the mask source
        sequence dimension should extend M.
      query_stream: Optional `Tensor`, the query stream. This is introduced in
        `TwoStreamRelativeAttention`/XLNet pretrainer. This is ignored if
        `two_stream` is `False`.
      query_attention_mask: Optional `Tensor` representing the mask that is
        added to query attention logits. If state is not None, the mask source
        sequence dimension should extend M.
      target_mapping: Optional `Tensor` representing the target mapping when
        calculating query attention.

    Returns:
      A `dict` object, containing the key value pairs for `content_attention`
      and (if `two_stream` is `True`) `query_attention`.

    """
    if not self._two_stream and query_stream is not None:
      logging.warning("`query_stream` was provided but two stream attention is "
                      "disabled. `query_stream` will be ignored.")
    if self._two_stream:
      attention_kwargs = dict(
          content_stream=content_stream,
          query_stream=query_stream,
          query_attention_mask=query_attention_mask,
          target_mapping=target_mapping,
          content_attention_mask=content_attention_mask)
    else:
      attention_kwargs = dict(
          query=content_stream,
          value=content_stream,
          key=content_stream,
          attention_mask=content_attention_mask)

    common_attention_kwargs = dict(
        content_attention_bias=content_attention_bias,
        relative_position_encoding=relative_position_encoding,
        positional_attention_bias=positional_attention_bias,
        segment_matrix=segment_matrix,
        segment_encoding=segment_encoding,
        segment_attention_bias=segment_attention_bias,
        state=state)

    attention_kwargs.update(common_attention_kwargs)
    attention_output = self._attention_layer(**attention_kwargs)

    if self._two_stream:
      attention_streams = attention_output
      input_streams = [content_stream, query_stream]
    else:
      attention_streams = [attention_output]
      input_streams = [content_stream]

    attention_keys = ["content_attention", "query_attention"]
    attention_output = {}
    for attention_stream, input_stream, attention_key in zip(
        attention_streams, input_streams, attention_keys):
      attention_stream = self._attention_dropout(attention_stream)
      attention_stream = self._attention_layer_norm(
          attention_stream + input_stream)
      inner_output = self._inner_dense(attention_stream)
      inner_output = self._inner_activation_layer(
          inner_output)
      inner_output = self._inner_dropout_layer(
          inner_output)
      layer_output = self._output_dense(inner_output)
      layer_output = self._output_dropout(layer_output)
      layer_output = self._output_layer_norm(layer_output + attention_stream)
      attention_output[attention_key] = layer_output

    return attention_output


class TransformerXL(tf.keras.layers.Layer):
  """Transformer XL.

  This layer combines multiple Transformer XL blocks from "Transformer-XL:
  Attentive Language Models Beyond a Fixed-Length Context"
  (https://arxiv.org/abs/1901.02860).

  This layer handles the attention biases as well as memory caching and reuse
  as in Transformer XL and XLNet.


  Attributes:
    vocab_size: The number of tokens in vocabulary.
    num_layers: The number of layers.
    hidden_size: The hidden size.
    num_attention_heads: The number of attention heads.
    head_size: The dimension size of each attention head.
    inner_size: The hidden size in feed-forward layers.
    dropout_rate: Dropout rate used in each Transformer XL block.
    attention_dropout_rate: Dropout rate on attention probabilities.
    two_stream: Whether or not to use `TwoStreamRelativeAttention` used
      in the XLNet pretrainer. If `False`, then it will use
      `MultiHeadRelativeAttention` as in Transformer XL.
    initializer: The initializer to use for attention biases.
    tie_attention_biases: Whether or not to tie biases together. If `True`, then
      each Transformer XL block shares the same trainable attention bias. If
      `False`, then each block has its own attention bias. This is usually set
      to `True`.
    memory_length: The number of tokens to cache.
    reuse_length: The number of tokens in the current batch to be cached
      and reused in the future.
    inner_activation: The activation to use in the inner layers
     for Transformer XL blocks. Typically "relu" or "gelu".
  """

  def __init__(self,
               vocab_size,
               num_layers,
               hidden_size,
               num_attention_heads,
               head_size,
               inner_size,
               dropout_rate,
               attention_dropout_rate,
               initializer,
               two_stream=False,
               tie_attention_biases=True,
               memory_length=None,
               reuse_length=None,
               inner_activation="relu",
               **kwargs):
    """Initializes TransformerXL."""
    super(TransformerXL, self).__init__(**kwargs)

    self._vocab_size = vocab_size
    self._initializer = initializer
    self._num_layers = num_layers
    self._hidden_size = hidden_size
    self._num_attention_heads = num_attention_heads
    self._head_size = head_size
    self._inner_size = inner_size
    self._inner_activation = inner_activation
    self._dropout_rate = dropout_rate
    self._attention_dropout_rate = attention_dropout_rate
    self._tie_attention_biases = tie_attention_biases
    self._two_stream = two_stream

    self._memory_length = memory_length
    self._reuse_length = reuse_length

    if self._tie_attention_biases:
      attention_bias_shape = [self._num_attention_heads, self._head_size]
    else:
      attention_bias_shape = [self._num_layers, self._num_attention_heads,
                              self._head_size]

    self.content_attention_bias = self.add_weight(
        "content_attention_bias",
        shape=attention_bias_shape,
        dtype=tf.float32,
        initializer=self._initializer)
    self.positional_attention_bias = self.add_weight(
        "positional_attention_bias",
        shape=attention_bias_shape,
        dtype=tf.float32,
        initializer=self._initializer)
    self.segment_attention_bias = self.add_weight(
        "segment_attention_bias",
        shape=attention_bias_shape,
        dtype=tf.float32,
        initializer=self._initializer)

    self.transformer_xl_layers = []
    for i in range(self._num_layers):
      self.transformer_xl_layers.append(
          TransformerXLBlock(
              vocab_size=self._vocab_size,
              hidden_size=self._head_size * self._num_attention_heads,
              num_attention_heads=self._num_attention_heads,
              head_size=self._head_size,
              inner_size=self._inner_size,
              dropout_rate=self._dropout_rate,
              attention_dropout_rate=self._attention_dropout_rate,
              norm_epsilon=1e-12,
              inner_activation=self._inner_activation,
              two_stream=self._two_stream,
              kernel_initializer="variance_scaling",
              name="layer_%d" % i))

    self.output_dropout = tf.keras.layers.Dropout(rate=self._dropout_rate)

  def get_config(self):
    config = {
        "vocab_size":
            self._vocab_size,
        "num_layers":
            self._num_layers,
        "hidden_size":
            self._hidden_size,
        "num_attention_heads":
            self._num_attention_heads,
        "head_size":
            self._head_size,
        "inner_size":
            self._inner_size,
        "dropout_rate":
            self._dropout_rate,
        "attention_dropout_rate":
            self._attention_dropout_rate,
        "initializer":
            self._initializer,
        "two_stream":
            self._two_stream,
        "tie_attention_biases":
            self._tie_attention_biases,
        "memory_length":
            self._memory_length,
        "reuse_length":
            self._reuse_length,
        "inner_activation":
            self._inner_activation,
    }
    base_config = super(TransformerXL, self).get_config()
    return dict(list(base_config.items()) + list(config.items()))

  def call(self,
           content_stream,
           relative_position_encoding,
           segment_matrix=None,
           segment_embedding=None,
           state=None,
           content_attention_mask=None,
           query_stream=None,
           query_attention_mask=None,
           target_mapping=None):
    """Implements call() for the layer.

    Args:
      content_stream: `Tensor`, the input content stream. This is the standard
        input to Transformer XL and is commonly referred to as `h` in XLNet.
      relative_position_encoding: Relative positional encoding `Tensor` of shape
        `[B, L, dim]`.
      segment_matrix: Optional `Tensor` of shape `[B, S, S + M]`. Used in XLNet,
        but not in Transformer XL.
      segment_embedding: Optional `Tensor` of shape `[2, num_heads, dim]`. Used
        in XLNet, but not in Transformer XL.
      state: Optional `Tensor` of shape `[B, M, E]`, where M is the length of
        the state or memory. If passed, this is also attended over as in
        Transformer XL.
      content_attention_mask: Optional `Tensor` representing the mask that is
        added to content attention logits. If state is not None, the mask source
        sequence dimension should extend M.
      query_stream: Optional `Tensor`, the query stream. This is introduced in
        `TwoStreamRelativeAttention`/XLNet pretrainer. This is ignored if
        `two_stream` is `False`.
      query_attention_mask: Optional `Tensor` representing the mask that is
        added to query attention logits. If state is not None, the mask source
        sequence dimension should extend M.
      target_mapping: Optional `Tensor` representing the target mapping when
        calculating query attention.

    Returns:
      A tuple consisting of the attention output and the list of cached memory
      states.
      The attention output is `content_attention` if `two_stream` is `False`,
      otherwise it is `query_attention`.
    """
    new_mems = []

    if state is None:
      state = [None] * self._num_layers
    for i in range(self._num_layers):
      # cache new mems
      new_mems.append(
          _cache_memory(content_stream, state[i],
                        self._memory_length, self._reuse_length))

      # segment bias
      if segment_matrix is None:
        segment_attention_bias = None
        segment_encoding = None
      else:
        segment_attention_bias = (self.segment_attention_bias
                                  if self._tie_attention_biases
                                  else self.segment_attention_bias[i])
        segment_encoding = segment_embedding[i]

      content_attention_bias = (self.content_attention_bias
                                if self._tie_attention_biases
                                else self.content_attention_bias[i])
      positional_attention_bias = (self.positional_attention_bias
                                   if self._tie_attention_biases
                                   else self.positional_attention_bias[i])
      transformer_xl_layer = self.transformer_xl_layers[i]
      transformer_xl_output = transformer_xl_layer(
          content_stream=content_stream,
          content_attention_bias=content_attention_bias,
          positional_attention_bias=positional_attention_bias,
          relative_position_encoding=relative_position_encoding,
          segment_matrix=segment_matrix,
          segment_encoding=segment_encoding,
          segment_attention_bias=segment_attention_bias,
          state=state[i],
          content_attention_mask=content_attention_mask,
          query_attention_mask=query_attention_mask,
          query_stream=query_stream,
          target_mapping=target_mapping)
      content_stream = transformer_xl_output["content_attention"]
      if self._two_stream:
        query_stream = transformer_xl_output["query_attention"]
      else:
        query_stream = None

    if self._two_stream:
      output_stream = query_stream
    else:
      output_stream = content_stream

    return output_stream, new_mems