bert_models.py

# Copyright 2019 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.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
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# ==============================================================================
"""BERT models that are compatible with TF 2.0."""

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function

import tensorflow as tf
import tensorflow_hub as hub

from official.modeling import tf_utils
from official.nlp import bert_modeling
from official.nlp.modeling import losses
from official.nlp.modeling import networks
from official.nlp.modeling.networks import bert_classifier
from official.nlp.modeling.networks import bert_pretrainer
from official.nlp.modeling.networks import bert_span_labeler


def gather_indexes(sequence_tensor, positions):
  """Gathers the vectors at the specific positions.

  Args:
      sequence_tensor: Sequence output of `BertModel` layer of shape
        (`batch_size`, `seq_length`, num_hidden) where num_hidden is number of
        hidden units of `BertModel` layer.
      positions: Positions ids of tokens in sequence to mask for pretraining of
        with dimension (batch_size, max_predictions_per_seq) where
        `max_predictions_per_seq` is maximum number of tokens to mask out and
        predict per each sequence.

  Returns:
      Masked out sequence tensor of shape (batch_size * max_predictions_per_seq,
      num_hidden).
  """
  sequence_shape = tf_utils.get_shape_list(
      sequence_tensor, name='sequence_output_tensor')
  batch_size = sequence_shape[0]
  seq_length = sequence_shape[1]
  width = sequence_shape[2]

  flat_offsets = tf.keras.backend.reshape(
      tf.range(0, batch_size, dtype=tf.int32) * seq_length, [-1, 1])
  flat_positions = tf.keras.backend.reshape(positions + flat_offsets, [-1])
  flat_sequence_tensor = tf.keras.backend.reshape(
      sequence_tensor, [batch_size * seq_length, width])
  output_tensor = tf.gather(flat_sequence_tensor, flat_positions)

  return output_tensor


class BertPretrainLossAndMetricLayer(tf.keras.layers.Layer):
  """Returns layer that computes custom loss and metrics for pretraining."""

  def __init__(self, vocab_size, **kwargs):
    super(BertPretrainLossAndMetricLayer, self).__init__(**kwargs)
    self._vocab_size = vocab_size
    self.config = {
        'vocab_size': vocab_size,
    }

  def _add_metrics(self, lm_output, lm_labels, lm_label_weights,
                   lm_example_loss, sentence_output, sentence_labels,
                   next_sentence_loss):
    """Adds metrics."""
    masked_lm_accuracy = tf.keras.metrics.sparse_categorical_accuracy(
        lm_labels, lm_output)
    numerator = tf.reduce_sum(masked_lm_accuracy * lm_label_weights)
    denominator = tf.reduce_sum(lm_label_weights) + 1e-5
    masked_lm_accuracy = numerator / denominator
    self.add_metric(
        masked_lm_accuracy, name='masked_lm_accuracy', aggregation='mean')

    self.add_metric(lm_example_loss, name='lm_example_loss', aggregation='mean')

    next_sentence_accuracy = tf.keras.metrics.sparse_categorical_accuracy(
        sentence_labels, sentence_output)
    self.add_metric(
        next_sentence_accuracy,
        name='next_sentence_accuracy',
        aggregation='mean')

    self.add_metric(
        next_sentence_loss, name='next_sentence_loss', aggregation='mean')

  def call(self, lm_output, sentence_output, lm_label_ids, lm_label_weights,
           sentence_labels):
    """Implements call() for the layer."""
    lm_label_weights = tf.keras.backend.cast(lm_label_weights, tf.float32)

    mask_label_loss = losses.weighted_sparse_categorical_crossentropy_loss(
        labels=lm_label_ids, predictions=lm_output, weights=lm_label_weights)
    sentence_loss = losses.weighted_sparse_categorical_crossentropy_loss(
        labels=sentence_labels, predictions=sentence_output)
    loss = mask_label_loss + sentence_loss
    batch_shape = tf.slice(tf.keras.backend.shape(sentence_labels), [0], [1])
    # TODO(hongkuny): Avoids the hack and switches add_loss.
    final_loss = tf.fill(batch_shape, loss)

    self._add_metrics(lm_output, lm_label_ids, lm_label_weights,
                      mask_label_loss, sentence_output, sentence_labels,
                      sentence_loss)
    return final_loss


def get_transformer_encoder(bert_config,
                            sequence_length,
                            float_dtype=tf.float32):
  """Gets a 'TransformerEncoder' object.

  Args:
    bert_config: A 'modeling.BertConfig' or 'modeling.AlbertConfig' object.
    sequence_length: Maximum sequence length of the training data.
    float_dtype: tf.dtype, tf.float32 or tf.float16.

  Returns:
    A networks.TransformerEncoder object.
  """
  kwargs = dict(
      vocab_size=bert_config.vocab_size,
      hidden_size=bert_config.hidden_size,
      num_layers=bert_config.num_hidden_layers,
      num_attention_heads=bert_config.num_attention_heads,
      intermediate_size=bert_config.intermediate_size,
      activation=tf_utils.get_activation(bert_config.hidden_act),
      dropout_rate=bert_config.hidden_dropout_prob,
      attention_dropout_rate=bert_config.attention_probs_dropout_prob,
      sequence_length=sequence_length,
      max_sequence_length=bert_config.max_position_embeddings,
      type_vocab_size=bert_config.type_vocab_size,
      initializer=tf.keras.initializers.TruncatedNormal(
          stddev=bert_config.initializer_range),
      float_dtype=float_dtype.name)
  if isinstance(bert_config, bert_modeling.AlbertConfig):
    kwargs['embedding_width'] = bert_config.embedding_size
    return networks.AlbertTransformerEncoder(**kwargs)
  else:
    assert isinstance(bert_config, bert_modeling.BertConfig)
    return networks.TransformerEncoder(**kwargs)


def pretrain_model(bert_config,
                   seq_length,
                   max_predictions_per_seq,
                   initializer=None):
  """Returns model to be used for pre-training.

  Args:
      bert_config: Configuration that defines the core BERT model.
      seq_length: Maximum sequence length of the training data.
      max_predictions_per_seq: Maximum number of tokens in sequence to mask out
        and use for pretraining.
      initializer: Initializer for weights in BertPretrainer.

  Returns:
      Pretraining model as well as core BERT submodel from which to save
      weights after pretraining.
  """
  input_word_ids = tf.keras.layers.Input(
      shape=(seq_length,), name='input_word_ids', dtype=tf.int32)
  input_mask = tf.keras.layers.Input(
      shape=(seq_length,), name='input_mask', dtype=tf.int32)
  input_type_ids = tf.keras.layers.Input(
      shape=(seq_length,), name='input_type_ids', dtype=tf.int32)
  masked_lm_positions = tf.keras.layers.Input(
      shape=(max_predictions_per_seq,),
      name='masked_lm_positions',
      dtype=tf.int32)
  masked_lm_ids = tf.keras.layers.Input(
      shape=(max_predictions_per_seq,), name='masked_lm_ids', dtype=tf.int32)
  masked_lm_weights = tf.keras.layers.Input(
      shape=(max_predictions_per_seq,),
      name='masked_lm_weights',
      dtype=tf.int32)
  next_sentence_labels = tf.keras.layers.Input(
      shape=(1,), name='next_sentence_labels', dtype=tf.int32)

  transformer_encoder = get_transformer_encoder(bert_config, seq_length)
  if initializer is None:
    initializer = tf.keras.initializers.TruncatedNormal(
        stddev=bert_config.initializer_range)
  pretrainer_model = bert_pretrainer.BertPretrainer(
      network=transformer_encoder,
      num_classes=2,  # The next sentence prediction label has two classes.
      num_token_predictions=max_predictions_per_seq,
      initializer=initializer,
      output='predictions')

  lm_output, sentence_output = pretrainer_model(
      [input_word_ids, input_mask, input_type_ids, masked_lm_positions])

  pretrain_loss_layer = BertPretrainLossAndMetricLayer(
      vocab_size=bert_config.vocab_size)
  output_loss = pretrain_loss_layer(lm_output, sentence_output, masked_lm_ids,
                                    masked_lm_weights, next_sentence_labels)
  keras_model = tf.keras.Model(
      inputs={
          'input_word_ids': input_word_ids,
          'input_mask': input_mask,
          'input_type_ids': input_type_ids,
          'masked_lm_positions': masked_lm_positions,
          'masked_lm_ids': masked_lm_ids,
          'masked_lm_weights': masked_lm_weights,
          'next_sentence_labels': next_sentence_labels,
      },
      outputs=output_loss)
  return keras_model, transformer_encoder


class BertSquadLogitsLayer(tf.keras.layers.Layer):
  """Returns a layer that computes custom logits for BERT squad model."""

  def __init__(self, initializer=None, float_type=tf.float32, **kwargs):
    super(BertSquadLogitsLayer, self).__init__(**kwargs)
    self.initializer = initializer
    self.float_type = float_type

  def build(self, unused_input_shapes):
    """Implements build() for the layer."""
    self.final_dense = tf.keras.layers.Dense(
        units=2, kernel_initializer=self.initializer, name='final_dense')
    super(BertSquadLogitsLayer, self).build(unused_input_shapes)

  def call(self, inputs):
    """Implements call() for the layer."""
    sequence_output = inputs

    input_shape = tf_utils.get_shape_list(
        sequence_output, name='sequence_output_tensor')
    sequence_length = input_shape[1]
    num_hidden_units = input_shape[2]

    final_hidden_input = tf.keras.backend.reshape(sequence_output,
                                                  [-1, num_hidden_units])
    logits = self.final_dense(final_hidden_input)
    logits = tf.keras.backend.reshape(logits, [-1, sequence_length, 2])
    logits = tf.transpose(logits, [2, 0, 1])
    unstacked_logits = tf.unstack(logits, axis=0)
    if self.float_type == tf.float16:
      unstacked_logits = tf.cast(unstacked_logits, tf.float32)
    return unstacked_logits[0], unstacked_logits[1]


def squad_model(bert_config,
                max_seq_length,
                float_type,
                initializer=None,
                hub_module_url=None):
  """Returns BERT Squad model along with core BERT model to import weights.

  Args:
    bert_config: BertConfig, the config defines the core Bert model.
    max_seq_length: integer, the maximum input sequence length.
    float_type: tf.dtype, tf.float32 or tf.bfloat16.
    initializer: Initializer for the final dense layer in the span labeler.
      Defaulted to TruncatedNormal initializer.
    hub_module_url: TF-Hub path/url to Bert module.

  Returns:
    A tuple of (1) keras model that outputs start logits and end logits and
    (2) the core BERT transformer encoder.
  """
  if initializer is None:
    initializer = tf.keras.initializers.TruncatedNormal(
        stddev=bert_config.initializer_range)
  if not hub_module_url:
    bert_encoder = get_transformer_encoder(bert_config, max_seq_length,
                                           float_type)
    return bert_span_labeler.BertSpanLabeler(
        network=bert_encoder, initializer=initializer), bert_encoder

  input_word_ids = tf.keras.layers.Input(
      shape=(max_seq_length,), dtype=tf.int32, name='input_word_ids')
  input_mask = tf.keras.layers.Input(
      shape=(max_seq_length,), dtype=tf.int32, name='input_mask')
  input_type_ids = tf.keras.layers.Input(
      shape=(max_seq_length,), dtype=tf.int32, name='input_type_ids')
  core_model = hub.KerasLayer(hub_module_url, trainable=True)
  _, sequence_output = core_model(
      [input_word_ids, input_mask, input_type_ids])

  squad_logits_layer = BertSquadLogitsLayer(
      initializer=initializer, float_type=float_type, name='squad_logits')
  start_logits, end_logits = squad_logits_layer(sequence_output)

  squad = tf.keras.Model(
      inputs={
          'input_word_ids': input_word_ids,
          'input_mask': input_mask,
          'input_type_ids': input_type_ids,
      },
      outputs=[start_logits, end_logits],
      name='squad_model')
  return squad, core_model


def classifier_model(bert_config,
                     float_type,
                     num_labels,
                     max_seq_length,
                     final_layer_initializer=None,
                     hub_module_url=None):
  """BERT classifier model in functional API style.

  Construct a Keras model for predicting `num_labels` outputs from an input with
  maximum sequence length `max_seq_length`.

  Args:
    bert_config: BertConfig or AlbertConfig, the config defines the core
      BERT or ALBERT model.
    float_type: dtype, tf.float32 or tf.bfloat16.
    num_labels: integer, the number of classes.
    max_seq_length: integer, the maximum input sequence length.
    final_layer_initializer: Initializer for final dense layer. Defaulted
      TruncatedNormal initializer.
    hub_module_url: TF-Hub path/url to Bert module.

  Returns:
    Combined prediction model (words, mask, type) -> (one-hot labels)
    BERT sub-model (words, mask, type) -> (bert_outputs)
  """
  if final_layer_initializer is not None:
    initializer = final_layer_initializer
  else:
    initializer = tf.keras.initializers.TruncatedNormal(
        stddev=bert_config.initializer_range)

  if not hub_module_url:
    bert_encoder = get_transformer_encoder(bert_config, max_seq_length)
    return bert_classifier.BertClassifier(
        bert_encoder,
        num_classes=num_labels,
        dropout_rate=bert_config.hidden_dropout_prob,
        initializer=initializer), bert_encoder

  input_word_ids = tf.keras.layers.Input(
      shape=(max_seq_length,), dtype=tf.int32, name='input_word_ids')
  input_mask = tf.keras.layers.Input(
      shape=(max_seq_length,), dtype=tf.int32, name='input_mask')
  input_type_ids = tf.keras.layers.Input(
      shape=(max_seq_length,), dtype=tf.int32, name='input_type_ids')
  bert_model = hub.KerasLayer(hub_module_url, trainable=True)
  pooled_output, _ = bert_model([input_word_ids, input_mask, input_type_ids])
  output = tf.keras.layers.Dropout(rate=bert_config.hidden_dropout_prob)(
      pooled_output)

  output = tf.keras.layers.Dense(
      num_labels,
      kernel_initializer=initializer,
      name='output',
      dtype=float_type)(
          output)
  return tf.keras.Model(
      inputs={
          'input_word_ids': input_word_ids,
          'input_mask': input_mask,
          'input_type_ids': input_type_ids
      },
      outputs=output), bert_model