谷歌BERT预训练源码解析（三）：训练过程

目录
前言
源码解析
主函数
自定义模型
遮蔽词预测
下一句预测
规范化数据集
前言
本部分介绍BERT训练过程，BERT模型训练过程是在自己的TPU上进行的，这部分我没做过研究所以不做深入探讨。BERT针对两个任务同时训练。1.下一句预测。2.遮蔽词识别
下面介绍BERT的预训练模型run_pretraining.py是怎么训练的。

源码解析
主函数
训练过程主要用了estimator调度器。这个调度器支持自定义训练过程，将训练集传入之后自动训练。详情见注释

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( #训练参数
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))

model_fn = model_fn_builder( #自定义模型，用于estimator训练
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)

# If TPU is not available, this will fall back to normal Estimator on CPU
# or GPU.
estimator = tf.contrib.tpu.TPUEstimator( #创建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: #训练过程
tf.logging.info("***** Running training *****")
tf.logging.info(" Batch size = %d", FLAGS.train_batch_size)
train_input_fn = input_fn_builder( #创建输入训练集
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: #验证过程
tf.logging.info("***** Running evaluation *****")
tf.logging.info(" Batch size = %d", FLAGS.eval_batch_size)

eval_input_fn = input_fn_builder( #创建验证集
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\n" % (key, str(result[key])))
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自定义模型
首先获取数据内容，传入到上一篇定义的模型中。对下一句预测任务取出模型的[CLS]结果。对遮蔽词预测任务取出模型的最后结果。然后分别计算loss值，最后将loss值相加。详情见注释

def model_fn_builder(bert_config, init_checkpoint, learning_rate,
num_train_steps, num_warmup_steps, use_tpu,
use_one_hot_embeddings):
"""Returns `model_fn` closure for TPUEstimator."""

def model_fn(features, labels, mode, params): # pylint: disable=unused-argument
"""The `model_fn` for TPUEstimator."""

tf.logging.info("*** Features ***")
for name in sorted(features.keys()):
tf.logging.info(" name = %s, shape = %s" % (name, features[name].shape))
#获取数据内容
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)
传入到Bert模型中。
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)
#遮蔽预测的batch_loss,平均loss，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)
#下一句预测的batch_loss,平均loss，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)
#loss相加
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
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 ****")
#如果有之前保存的模型
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
if mode == tf.estimator.ModeKeys.TRAIN:
train_op = optimization.create_optimizer( #自定义好的优化器
total_loss, learning_rate, num_train_steps, num_warmup_steps, use_tpu)

output_spec = tf.contrib.tpu.TPUEstimatorSpec( #Estimator要求返回一个EstimatorSpec对象
mode=mode,
loss=total_loss,
train_op=train_op,
scaffold_fn=scaffold_fn)
#验证过程
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):
"""Computes the loss and accuracy of the model."""
masked_lm_log_probs = tf.reshape(masked_lm_log_probs,
[-1, masked_lm_log_probs.shape[-1]]) #概率矩阵转成[batch_size*max_pred_pre_seq,vocab_size]
masked_lm_predictions = tf.argmax(
masked_lm_log_probs, axis=-1, output_type=tf.int32) #取最大值位置为输出
masked_lm_example_loss = tf.reshape(masked_lm_example_loss, [-1]) #每句loss列表 [batch_size*max_pred_per_seq]
masked_lm_ids = tf.reshape(masked_lm_ids, [-1])
masked_lm_weights = tf.reshape(masked_lm_weights, [-1])
masked_lm_accuracy = tf.metrics.accuracy( #计算准确率
labels=masked_lm_ids,
predictions=masked_lm_predictions,
weights=masked_lm_weights)
masked_lm_mean_loss = tf.metrics.mean( #计算平均loss
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( #获取最大位置为输出
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( #计算准确率
labels=next_sentence_labels, predictions=next_sentence_predictions)
next_sentence_mean_loss = tf.metrics.mean( 计算平均loss
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( #Estimator要求返回一个EstimatorSpec对象
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
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遮蔽词预测
输入BERT模型的最后一层encoder，输出遮蔽词预测任务的loss和概率矩阵。详情见注释

def get_masked_lm_output(bert_config, input_tensor, output_weights, positions,
label_ids, label_weights):
#这里的input_tensor是模型中传回的最后一层结果 [batch_size,seq_length,hidden_size]。
#output_weights是词向量表 [vocab_size,embedding_size]
"""Get loss and log probs for the masked LM."""
#获取positions位置的所有encoder（即要预测的那些位置的encoder）
input_tensor = gather_indexes(input_tensor, positions) #[batch_size*max_pred_pre_seq,hidden_size]

with tf.variable_scope("cls/predictions"):
# We apply one more non-linear transformation before the output layer.
# This matrix is not used after pre-training.
with tf.variable_scope("transform"):
input_tensor = tf.layers.dense( #传入一个全连接层 输出shape [batch_size*max_pred_pre_seq,hidden_size]
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)

# The output weights are the same as the input embeddings, but there is
# an output-only bias for each token.
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) #[batch_size*max_pred_pre_seq,vocab_size]
logits = tf.nn.bias_add(logits, output_bias) #加bias
log_probs = tf.nn.log_softmax(logits, axis=-1) #[batch_size*max_pred_pre_seq,vocab_size]

label_ids = tf.reshape(label_ids, [-1]) #[batch_size*max_pred_per_seq]
label_weights = tf.reshape(label_weights, [-1])

one_hot_labels = tf.one_hot( #[batch_size*max_pred_per_seq,vocab_size]
label_ids, depth=bert_config.vocab_size, dtype=tf.float32) #label id转one hot

# The `positions` tensor might be zero-padded (if the sequence is too
# short to have the maximum number of predictions). The `label_weights`
# tensor has a value of 1.0 for every real prediction and 0.0 for the
# padding predictions.
per_example_loss = -tf.reduce_sum(log_probs * one_hot_labels, axis=[-1]) #[batch_size*max_pred_per_seq]
numerator = tf.reduce_sum(label_weights * per_example_loss) #[1] 一个batch的loss
denominator = tf.reduce_sum(label_weights) + 1e-5
loss = numerator / denominator #平均loss

return (loss, per_example_loss, log_probs)
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下一句预测
输入BERT模型CLS的encoder，输出下一句预测任务的loss和概率矩阵,详情见注释

def get_next_sentence_output(bert_config, input_tensor, labels):
#input_tensor shape [batch_size,hidden_size]
"""Get loss and log probs for the next sentence prediction."""

# Simple binary classification. Note that 0 is "next sentence" and 1 is
# "random sentence". This weight matrix is not used after pre-training.
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()) #[batch_size,hidden_size]

logits = tf.matmul(input_tensor, output_weights, transpose_b=True) #[batch_size,2]
logits = tf.nn.bias_add(logits, output_bias) #[batch_size,2]
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) #[batch_size,2]
per_example_loss = -tf.reduce_sum(one_hot_labels * log_probs, axis=-1) #[batch_size]
loss = tf.reduce_mean(per_example_loss) #[1]
return (loss, per_example_loss, log_probs)
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规范化数据集
Estimator要求模型的输入为特定格式(from_tensor_slices)，所以要对数据进行类封装

def input_fn_builder(input_files,
max_seq_length,
max_predictions_per_seq,
is_training,
num_cpu_threads=4):
"""Creates an `input_fn` closure to be passed to TPUEstimator."""

def input_fn(params):
"""The actual input function."""
batch_size = params["batch_size"]

name_to_features = {
"input_ids":
tf.FixedLenFeature([max_seq_length], tf.int64),
"input_mask":
tf.FixedLenFeature([max_seq_length], tf.int64),
"segment_ids":
tf.FixedLenFeature([max_seq_length], tf.int64),
"masked_lm_positions":
tf.FixedLenFeature([max_predictions_per_seq], tf.int64),
"masked_lm_ids":
tf.FixedLenFeature([max_predictions_per_seq], tf.int64),
"masked_lm_weights":
tf.FixedLenFeature([max_predictions_per_seq], tf.float32),
"next_sentence_labels":
tf.FixedLenFeature([1], tf.int64),
}

# For training, we want a lot of parallel reading and shuffling.
# For eval, we want no shuffling and parallel reading doesn't matter.
if is_training:
d = tf.data.Dataset.from_tensor_slices(tf.constant(input_files))
d = d.repeat() #重复
d = d.shuffle(buffer_size=len(input_files)) #打乱

# `cycle_length` is the number of parallel files that get read.
cycle_length = min(num_cpu_threads, len(input_files))

# `sloppy` mode means that the interleaving is not exact. This adds
# even more randomness to the training pipeline.
d = d.apply(
tf.contrib.data.parallel_interleave( #生成嵌套数据集，并且输出其元素隔行交错
tf.data.TFRecordDataset,
sloppy=is_training,
cycle_length=cycle_length))
d = d.shuffle(buffer_size=100)
else:
d = tf.data.TFRecordDataset(input_files)
# Since we evaluate for a fixed number of steps we don't want to encounter
# out-of-range exceptions.
d = d.repeat()

# We must `drop_remainder` on training because the TPU requires fixed
# size dimensions. For eval, we assume we are evaluating on the CPU or GPU
# and we *don't* want to drop the remainder, otherwise we wont cover
# every sample.
d = d.apply(
tf.contrib.data.map_and_batch( #结构转换
lambda record: _decode_record(record, name_to_features),
batch_size=batch_size,
num_parallel_batches=num_cpu_threads,
drop_remainder=True))
return d

return input_fn
---------------------
作者：保持一份率性
来源：CSDN
原文：https://blog.csdn.net/weixin_39470744/article/details/84619903
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