BERTBERT源码分析（PART III）

Posted 2021-04-05 机器学习算法与自然语言处理

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写在前面

继续之前没有介绍完的 Pre-training 部分，在上一篇中()我们已经完成了对输入数据的处理，接下来看看 BERT 是怎么完成「Masked LM」和「Next Sentence Prediction」两个任务的训练。

• run_pretraining ^[1]

除了代码块外部，在内部也有注释噢。之前代码黑色背景好像有点不舒服，换成白色试试

另外，把BERT源码分析系列整理成了PDF版本方便阅读，有需要的可以在文末获取（别急着拉到下面，先看完这篇）

任务#1：Masked LM

get_masked_lm_output函数用于计算「任务#1」的训练 loss。输入为 BertModel 的最后一层 sequence_output 输出（[batch_size, seq_length, hidden_size]），因为对一个序列的 MASK 标记的预测属于标注问题，需要整个 sequence 的输出状态。

def get_masked_lm_output(bert_config, input_tensor, output_weights, positions,
 label_ids, label_weights):
  """Get loss and log probs for the masked LM."""
  # 获取mask词的encode
  input_tensor = gather_indexes(input_tensor, positions)

  with tf.variable_scope("cls/predictions"):
    # 在输出之前添加一个非线性变换，只在预训练阶段起作用
    with tf.variable_scope("transform"):
      input_tensor = tf.layers.dense(
          input_tensor,
          units=bert_config.hidden_size,
          activation=modeling.get_activation(bert_config.hidden_act),
          kernel_initializer=modeling.create_initializer(
              bert_config.initializer_range))
      input_tensor = modeling.layer_norm(input_tensor)

    # output_weights是和传入的word embedding一样的
    # 这里再添加一个bias
    output_bias = tf.get_variable(
        "output_bias",
        shape=[bert_config.vocab_size],
        initializer=tf.zeros_initializer())
    logits = tf.matmul(input_tensor, output_weights, transpose_b=True)
    logits = tf.nn.bias_add(logits, output_bias)
    log_probs = tf.nn.log_softmax(logits, axis=-1)

	# label_ids表示mask掉的Token的id
    label_ids = tf.reshape(label_ids, [-1])
    label_weights = tf.reshape(label_weights, [-1])

    one_hot_labels = tf.one_hot(
        label_ids, depth=bert_config.vocab_size, dtype=tf.float32)

    # 但是由于实际MASK的可能不到20，比如只MASK18，那么label_ids有2个0(padding)
    # 而label_weights=[1, 1, ...., 0, 0]，说明后面两个label_id是padding的，计算loss要去掉。
    per_example_loss = -tf.reduce_sum(log_probs * one_hot_labels, axis=[-1])
    numerator = tf.reduce_sum(label_weights * per_example_loss)
    denominator = tf.reduce_sum(label_weights) + 1e-5
    loss = numerator / denominator

  return (loss, per_example_loss, log_probs)

任务#2 Next Sentence Prediction

get_next_sentence_output函数用于计算「任务#2」的训练 loss。输入为 BertModel 的最后一层 pooled_output 输出（[batch_size, hidden_size]），因为该任务属于二分类问题，所以只需要每个序列的第一个 token【CLS】即可。

def get_next_sentence_output(bert_config, input_tensor, labels):
  """Get loss and log probs for the next sentence prediction."""

 # 标签0表示 下一个句子关系成立；标签1表示 下一个句子关系不成立。
 # 这个分类器的参数在实际Fine-tuning阶段会丢弃掉
  with tf.variable_scope("cls/seq_relationship"):
    output_weights = tf.get_variable(
        "output_weights",
        shape=[2, bert_config.hidden_size],
        initializer=modeling.create_initializer(bert_config.initializer_range))
    output_bias = tf.get_variable(
        "output_bias", shape=[2], initializer=tf.zeros_initializer())

    logits = tf.matmul(input_tensor, output_weights, transpose_b=True)
    logits = tf.nn.bias_add(logits, output_bias)
    log_probs = tf.nn.log_softmax(logits, axis=-1)
    labels = tf.reshape(labels, [-1])
    one_hot_labels = tf.one_hot(labels, depth=2, dtype=tf.float32)
    per_example_loss = -tf.reduce_sum(one_hot_labels * log_probs, axis=-1)
    loss = tf.reduce_mean(per_example_loss)
    return (loss, per_example_loss, log_probs)

自定义模型

module_fn_builder函数，用于构造 Estimator 使用的model_fn。定义好了上述两个训练任务，就可以写出训练过程，之后将训练集传入自动训练。

def model_fn_builder(bert_config, init_checkpoint, learning_rate,
 num_train_steps, num_warmup_steps, use_tpu,
 use_one_hot_embeddings):

  def model_fn(features, labels, mode, params):

    tf.logging.info("*** Features ***")
    for name in sorted(features.keys()):
      tf.logging.info(" name = %s, shape = %s" % (name, features[name].shape))

    input_ids = features["input_ids"]
    input_mask = features["input_mask"]
    segment_ids = features["segment_ids"]
    masked_lm_positions = features["masked_lm_positions"]
    masked_lm_ids = features["masked_lm_ids"]
    masked_lm_weights = features["masked_lm_weights"]
    next_sentence_labels = features["next_sentence_labels"]

    is_training = (mode == tf.estimator.ModeKeys.TRAIN)

	# 创建Transformer实例对象
    model = modeling.BertModel(
        config=bert_config,
        is_training=is_training,
        input_ids=input_ids,
        input_mask=input_mask,
        token_type_ids=segment_ids,
        use_one_hot_embeddings=use_one_hot_embeddings)

	# 获得MASK LM任务的批损失，平均损失以及预测概率矩阵
    (masked_lm_loss,
     masked_lm_example_loss, masked_lm_log_probs) = get_masked_lm_output(
         bert_config, model.get_sequence_output(), model.get_embedding_table(),
         masked_lm_positions, masked_lm_ids, masked_lm_weights)

	# 获得NEXT SENTENCE PREDICTION任务的批损失，平均损失以及预测概率矩阵
    (next_sentence_loss, next_sentence_example_loss,
     next_sentence_log_probs) = get_next_sentence_output(
         bert_config, model.get_pooled_output(), next_sentence_labels)

	# 总的损失定义为两者之和
    total_loss = masked_lm_loss + next_sentence_loss

	# 获取所有变量
    tvars = tf.trainable_variables()

    initialized_variable_names = {}
    scaffold_fn = None
    # 如果有之前保存的模型，则进行恢复
    if init_checkpoint:
      (assignment_map, initialized_variable_names
      ) = modeling.get_assignment_map_from_checkpoint(tvars, init_checkpoint)
      if use_tpu:

        def tpu_scaffold():
          tf.train.init_from_checkpoint(init_checkpoint, assignment_map)
          return tf.train.Scaffold()

        scaffold_fn = tpu_scaffold
      else:
        tf.train.init_from_checkpoint(init_checkpoint, assignment_map)

    tf.logging.info("**** Trainable Variables ****")
    for var in tvars:
      init_string = ""
      if var.name in initialized_variable_names:
        init_string = ", *INIT_FROM_CKPT*"
      tf.logging.info(" name = %s, shape = %s%s", var.name, var.shape,
                      init_string)

    output_spec = None
    # 训练过程，获得spec
    if mode == tf.estimator.ModeKeys.TRAIN:
      train_op = optimization.create_optimizer(
          total_loss, learning_rate, num_train_steps, num_warmup_steps, use_tpu)

      output_spec = tf.contrib.tpu.TPUEstimatorSpec(
          mode=mode,
          loss=total_loss,
          train_op=train_op,
          scaffold_fn=scaffold_fn)
    # 验证过程spec
    elif mode == tf.estimator.ModeKeys.EVAL:

      def metric_fn(masked_lm_example_loss, masked_lm_log_probs, masked_lm_ids,
 masked_lm_weights, next_sentence_example_loss,
 next_sentence_log_probs, next_sentence_labels):
        """计算损失和准确率"""
        masked_lm_log_probs = tf.reshape(masked_lm_log_probs,
                                         [-1, masked_lm_log_probs.shape[-1]])
        masked_lm_predictions = tf.argmax(
            masked_lm_log_probs, axis=-1, output_type=tf.int32)
        masked_lm_example_loss = tf.reshape(masked_lm_example_loss, [-1])
        masked_lm_ids = tf.reshape(masked_lm_ids, [-1])
        masked_lm_weights = tf.reshape(masked_lm_weights, [-1])
        masked_lm_accuracy = tf.metrics.accuracy(
            labels=masked_lm_ids,
            predictions=masked_lm_predictions,
            weights=masked_lm_weights)
        masked_lm_mean_loss = tf.metrics.mean(
            values=masked_lm_example_loss, weights=masked_lm_weights)

        next_sentence_log_probs = tf.reshape(
            next_sentence_log_probs, [-1, next_sentence_log_probs.shape[-1]])
        next_sentence_predictions = tf.argmax(
            next_sentence_log_probs, axis=-1, output_type=tf.int32)
        next_sentence_labels = tf.reshape(next_sentence_labels, [-1])
        next_sentence_accuracy = tf.metrics.accuracy(
            labels=next_sentence_labels, predictions=next_sentence_predictions)
        next_sentence_mean_loss = tf.metrics.mean(
            values=next_sentence_example_loss)

        return {
            "masked_lm_accuracy": masked_lm_accuracy,
            "masked_lm_loss": masked_lm_mean_loss,
            "next_sentence_accuracy": next_sentence_accuracy,
            "next_sentence_loss": next_sentence_mean_loss,
        }

      eval_metrics = (metric_fn, [
          masked_lm_example_loss, masked_lm_log_probs, masked_lm_ids,
          masked_lm_weights, next_sentence_example_loss,
          next_sentence_log_probs, next_sentence_labels
      ])
      output_spec = tf.contrib.tpu.TPUEstimatorSpec(
          mode=mode,
          loss=total_loss,
          eval_metrics=eval_metrics,
          scaffold_fn=scaffold_fn)
    else:
      raise ValueError("Only TRAIN and EVAL modes are supported: %s" % (mode))

    return output_spec

  return model_fn

主函数

基于上述函数实现训练过程

def main(_):
  tf.logging.set_verbosity(tf.logging.INFO)
  if not FLAGS.do_train and not FLAGS.do_eval:
    raise ValueError("At least one of `do_train` or `do_eval` must be True.")
  bert_config = modeling.BertConfig.from_json_file(FLAGS.bert_config_file)
  tf.gfile.MakeDirs(FLAGS.output_dir)

  input_files = []
  for input_pattern in FLAGS.input_file.split(","):
    input_files.extend(tf.gfile.Glob(input_pattern))

  tf.logging.info("*** Input Files ***")
  for input_file in input_files:
    tf.logging.info(" %s" % input_file)

  tpu_cluster_resolver = None
  if FLAGS.use_tpu and FLAGS.tpu_name:
    tpu_cluster_resolver = tf.contrib.cluster_resolver.TPUClusterResolver(
        FLAGS.tpu_name, zone=FLAGS.tpu_zone, project=FLAGS.gcp_project)

  is_per_host = tf.contrib.tpu.InputPipelineConfig.PER_HOST_V2
  run_config = tf.contrib.tpu.RunConfig(
      cluster=tpu_cluster_resolver,
      master=FLAGS.master,
      model_dir=FLAGS.output_dir,
      save_checkpoints_steps=FLAGS.save_checkpoints_steps,
      tpu_config=tf.contrib.tpu.TPUConfig(
          iterations_per_loop=FLAGS.iterations_per_loop,
          num_shards=FLAGS.num_tpu_cores,
          per_host_input_for_training=is_per_host))

  # 自定义模型用于estimator训练
  model_fn = model_fn_builder(
      bert_config=bert_config,
      init_checkpoint=FLAGS.init_checkpoint,
      learning_rate=FLAGS.learning_rate,
      num_train_steps=FLAGS.num_train_steps,
      num_warmup_steps=FLAGS.num_warmup_steps,
      use_tpu=FLAGS.use_tpu,
      use_one_hot_embeddings=FLAGS.use_tpu)

  # 如果没有TPU，会自动转为CPU/GPU的Estimator
  estimator = tf.contrib.tpu.TPUEstimator(
      use_tpu=FLAGS.use_tpu,
      model_fn=model_fn,
      config=run_config,
      train_batch_size=FLAGS.train_batch_size,
      eval_batch_size=FLAGS.eval_batch_size)

  if FLAGS.do_train:
    tf.logging.info("***** Running training *****")
    tf.logging.info(" Batch size = %d", FLAGS.train_batch_size)
    train_input_fn = input_fn_builder(
        input_files=input_files,
        max_seq_length=FLAGS.max_seq_length,
        max_predictions_per_seq=FLAGS.max_predictions_per_seq,
        is_training=True)
    estimator.train(input_fn=train_input_fn, max_steps=FLAGS.num_train_steps)

  if FLAGS.do_eval:
    tf.logging.info("***** Running evaluation *****")
    tf.logging.info(" Batch size = %d", FLAGS.eval_batch_size)

    eval_input_fn = input_fn_builder(
        input_files=input_files,
        max_seq_length=FLAGS.max_seq_length,
        max_predictions_per_seq=FLAGS.max_predictions_per_seq,
        is_training=False)

    result = estimator.evaluate(
        input_fn=eval_input_fn, steps=FLAGS.max_eval_steps)

    output_eval_file = os.path.join(FLAGS.output_dir, "eval_results.txt")
    with tf.gfile.GFile(output_eval_file, "w") as writer:
      tf.logging.info("***** Eval results *****")
      for key in sorted(result.keys()):
        tf.logging.info(" %s = %s", key, str(result[key]))
        writer.write("%s = %s
" % (key, str(result[key])))

代码测试

预训练运行脚本

python run_pretraining.py 
  --input_file=/tmp/tf_examples.tfrecord 
  --output_dir=/tmp/pretraining_output 
  --do_train=True 
  --do_eval=True 
  --bert_config_file=$BERT_BASE_DIR/bert_config.json 
  --init_checkpoint=$BERT_BASE_DIR/bert_model.ckpt 
  --train_batch_size=32 
  --max_seq_length=128 
  --max_predictions_per_seq=20 
  --num_train_steps=20 
  --num_warmup_steps=10 
  --learning_rate=2e-5

之后你可以得到类似以下输出日志：

***** Eval results *****
  global_step = 20
  loss = 0.0979674
  masked_lm_accuracy = 0.985479
  masked_lm_loss = 0.0979328
  next_sentence_accuracy = 1.0
  next_sentence_loss = 3.45724e-05

最后贴一个预训练过程的 tips【反正我也做不了，看看就行= 。=】

---

Over~BERT源码系列到这里就结束啦。

PS.到现在为止，BERT也更新了很多比如Whole Word Masking等等，所以之前有错误的还请大家一定指出，我好及时修正~

本文参考资料

[1]

run_pretraining: https://github.com/google-research/bert/blob/master/run_pretraining.py

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