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"""Masked language task."""

import dataclasses
import tensorflow as tf, tf_keras

from official.core import base_task
from official.core import config_definitions as cfg
from official.core import task_factory
from official.modeling import tf_utils
from official.nlp.configs import bert
from official.nlp.configs import encoders
from official.nlp.data import data_loader_factory
from official.nlp.modeling import layers
from official.nlp.modeling import models


@dataclasses.dataclass
class MaskedLMConfig(cfg.TaskConfig):
  """The model config."""
  model: bert.PretrainerConfig = dataclasses.field(
      default_factory=lambda: bert.PretrainerConfig(  # pylint: disable=g-long-lambda
          cls_heads=[
              bert.ClsHeadConfig(
                  inner_dim=768,
                  num_classes=2,
                  dropout_rate=0.1,
                  name='next_sentence',
              )
          ]
      )
  )
  # TODO(b/154564893): Mathematically, scale_loss should be True.
  # However, it works better with scale_loss being False.
  scale_loss: bool = False
  train_data: cfg.DataConfig = dataclasses.field(default_factory=cfg.DataConfig)
  validation_data: cfg.DataConfig = dataclasses.field(
      default_factory=cfg.DataConfig
  )


@task_factory.register_task_cls(MaskedLMConfig)
class MaskedLMTask(base_task.Task):
  """Task object for Mask language modeling."""

  def _build_encoder(self, encoder_cfg):
    return encoders.build_encoder(encoder_cfg)

  def build_model(self, params=None):
    config = params or self.task_config.model
    encoder_cfg = config.encoder
    encoder_network = self._build_encoder(encoder_cfg)
    cls_heads = [
        layers.ClassificationHead(**cfg.as_dict()) for cfg in config.cls_heads
    ] if config.cls_heads else []
    return models.BertPretrainerV2(
        mlm_activation=tf_utils.get_activation(config.mlm_activation),
        mlm_initializer=tf_keras.initializers.TruncatedNormal(
            stddev=config.mlm_initializer_range),
        encoder_network=encoder_network,
        classification_heads=cls_heads)

  def build_losses(self,
                   labels,
                   model_outputs,
                   metrics,
                   aux_losses=None) -> tf.Tensor:
    with tf.name_scope('MaskedLMTask/losses'):
      metrics = dict([(metric.name, metric) for metric in metrics])
      lm_prediction_losses = tf_keras.losses.sparse_categorical_crossentropy(
          labels['masked_lm_ids'],
          tf.cast(model_outputs['mlm_logits'], tf.float32),
          from_logits=True)
      lm_label_weights = labels['masked_lm_weights']
      lm_numerator_loss = tf.reduce_sum(lm_prediction_losses *
                                        lm_label_weights)
      lm_denominator_loss = tf.reduce_sum(lm_label_weights)
      mlm_loss = tf.math.divide_no_nan(lm_numerator_loss, lm_denominator_loss)
      metrics['lm_example_loss'].update_state(mlm_loss)
      if 'next_sentence_labels' in labels:
        sentence_labels = labels['next_sentence_labels']
        sentence_outputs = tf.cast(
            model_outputs['next_sentence'], dtype=tf.float32)
        sentence_loss = tf.reduce_mean(
            tf_keras.losses.sparse_categorical_crossentropy(
                sentence_labels, sentence_outputs, from_logits=True))
        metrics['next_sentence_loss'].update_state(sentence_loss)
        total_loss = mlm_loss + sentence_loss
      else:
        total_loss = mlm_loss

      if aux_losses:
        total_loss += tf.add_n(aux_losses)
      return total_loss

  def build_inputs(self, params, input_context=None):
    """Returns tf.data.Dataset for pretraining."""
    if params.input_path == 'dummy':

      def dummy_data(_):
        dummy_ids = tf.zeros((1, params.seq_length), dtype=tf.int32)
        dummy_lm = tf.zeros((1, params.max_predictions_per_seq), dtype=tf.int32)
        return dict(
            input_word_ids=dummy_ids,
            input_mask=dummy_ids,
            input_type_ids=dummy_ids,
            masked_lm_positions=dummy_lm,
            masked_lm_ids=dummy_lm,
            masked_lm_weights=tf.cast(dummy_lm, dtype=tf.float32),
            next_sentence_labels=tf.zeros((1, 1), dtype=tf.int32))

      dataset = tf.data.Dataset.range(1)
      dataset = dataset.repeat()
      dataset = dataset.map(
          dummy_data, num_parallel_calls=tf.data.experimental.AUTOTUNE)
      return dataset

    return data_loader_factory.get_data_loader(params).load(input_context)

  def build_metrics(self, training=None):
    del training
    metrics = [
        tf_keras.metrics.SparseCategoricalAccuracy(name='masked_lm_accuracy'),
        tf_keras.metrics.Mean(name='lm_example_loss')
    ]
    # TODO(hongkuny): rethink how to manage metrics creation with heads.
    if self.task_config.train_data.use_next_sentence_label:
      metrics.append(
          tf_keras.metrics.SparseCategoricalAccuracy(
              name='next_sentence_accuracy'))
      metrics.append(tf_keras.metrics.Mean(name='next_sentence_loss'))
    return metrics

  def process_metrics(self, metrics, labels, model_outputs):
    with tf.name_scope('MaskedLMTask/process_metrics'):
      metrics = dict([(metric.name, metric) for metric in metrics])
      if 'masked_lm_accuracy' in metrics:
        metrics['masked_lm_accuracy'].update_state(
            labels['masked_lm_ids'], model_outputs['mlm_logits'],
            labels['masked_lm_weights'])
      if 'next_sentence_accuracy' in metrics:
        metrics['next_sentence_accuracy'].update_state(
            labels['next_sentence_labels'], model_outputs['next_sentence'])

  def train_step(self, inputs, model: tf_keras.Model,
                 optimizer: tf_keras.optimizers.Optimizer, metrics):
    """Does forward and backward.

    Args:
      inputs: a dictionary of input tensors.
      model: the model, forward pass definition.
      optimizer: the optimizer for this training step.
      metrics: a nested structure of metrics objects.

    Returns:
      A dictionary of logs.
    """
    with tf.GradientTape() as tape:
      outputs = model(inputs, training=True)
      # Computes per-replica loss.
      loss = self.build_losses(
          labels=inputs,
          model_outputs=outputs,
          metrics=metrics,
          aux_losses=model.losses)
      if self.task_config.scale_loss:
        # Scales loss as the default gradients allreduce performs sum inside the
        # optimizer.
        scaled_loss = loss / tf.distribute.get_strategy().num_replicas_in_sync
    tvars = model.trainable_variables
    if self.task_config.scale_loss:
      grads = tape.gradient(scaled_loss, tvars)
    else:
      grads = tape.gradient(loss, tvars)
    optimizer.apply_gradients(list(zip(grads, tvars)))
    self.process_metrics(metrics, inputs, outputs)
    return {self.loss: loss}

  def validation_step(self, inputs, model: tf_keras.Model, metrics):
    """Validatation step.

    Args:
      inputs: a dictionary of input tensors.
      model: the keras.Model.
      metrics: a nested structure of metrics objects.

    Returns:
      A dictionary of logs.
    """
    outputs = self.inference_step(inputs, model)
    loss = self.build_losses(
        labels=inputs,
        model_outputs=outputs,
        metrics=metrics,
        aux_losses=model.losses)
    self.process_metrics(metrics, inputs, outputs)
    return {self.loss: loss}
