本篇文章主要是解读模型主体代码modeling.py。在阅读这篇文章之前希望读者们对bert的相关理论有一定的了解，尤其是transformer的结构原理，网上的资料很多，本文内容对原理部分就不做过多的介绍了。

我自己写出来其中一个目的也是帮助自己学习整理、当你输出的时候才也会明白哪里懂了哪里不懂。因为水平有限，很多地方理解不到位的，还请各位批评指正。

1、配置

class BertConfig(object):"""Configuration for `BertModel`."""def __init__(self,vocab_size,hidden_size=768,num_hidden_layers=12,num_attention_heads=12,intermediate_size=3072,hidden_act="gelu",hidden_dropout_prob=0.1,attention_probs_dropout_prob=0.1,max_position_embeddings=512,type_vocab_size=16,initializer_range=0.02):self.vocab_size = vocab_sizeself.hidden_size = hidden_sizeself.num_hidden_layers = num_hidden_layersself.num_attention_heads = num_attention_headsself.hidden_act = hidden_actself.intermediate_size = intermediate_sizeself.hidden_dropout_prob = hidden_dropout_probself.attention_probs_dropout_prob = attention_probs_dropout_probself.max_position_embeddings = max_position_embeddingsself.type_vocab_size = type_vocab_sizeself.initializer_range = initializer_range

模型配置，比较简单，依次是：词典大小、隐层神经元个数、transformer的层数、attention的头数、激活函数、中间层神经元个数、隐层dropout比例、attention里面dropout比例、sequence最大长度、token_type_ids的词典大小、truncated_normal_initializer的stdev。

2、word embedding

def embedding_lookup(input_ids,vocab_size,embedding_size=128,initializer_range=0.02,word_embedding_name="word_embeddings",use_one_hot_embeddings=False):if input_ids.shape.ndims == 2:input_ids = tf.expand_dims(input_ids, axis=[-1])embedding_table = tf.get_variable(name=word_embedding_name,shape=[vocab_size, embedding_size],initializer=create_initializer(initializer_range))if use_one_hot_embeddings:flat_input_ids = tf.reshape(input_ids, [-1])one_hot_input_ids = tf.one_hot(flat_input_ids, depth=vocab_size)output = tf.matmul(one_hot_input_ids, embedding_table)else:output = tf.nn.embedding_lookup(embedding_table, input_ids)input_shape = get_shape_list(input_ids)output = tf.reshape(output,input_shape[0:-1] + [input_shape[-1] * embedding_size])return (output, embedding_table)

构造embedding_table，进行word embedding，可选one_hot的方式，返回embedding的结果和embedding_table

3、词向量的后续处理

def embedding_postprocessor(input_tensor,use_token_type=False,token_type_ids=None,token_type_vocab_size=16,token_type_embedding_name="token_type_embeddings",use_position_embeddings=True,position_embedding_name="position_embeddings",initializer_range=0.02,max_position_embeddings=512,dropout_prob=0.1):input_shape = get_shape_list(input_tensor, expected_rank=3)batch_size = input_shape[0]seq_length = input_shape[1]width = input_shape[2]output = input_tensorif use_token_type:if token_type_ids is None:raise ValueError("`token_type_ids` must be specified if""`use_token_type` is True.")token_type_table = tf.get_variable(name=token_type_embedding_name,shape=[token_type_vocab_size, width],initializer=create_initializer(initializer_range))flat_token_type_ids = tf.reshape(token_type_ids, [-1])one_hot_ids = tf.one_hot(flat_token_type_ids, depth=token_type_vocab_size)token_type_embeddings = tf.matmul(one_hot_ids, token_type_table)token_type_embeddings = tf.reshape(token_type_embeddings,[batch_size, seq_length, width])output += token_type_embeddingsif use_position_embeddings:assert_op = tf.assert_less_equal(seq_length, max_position_embeddings)with tf.control_dependencies([assert_op]):full_position_embeddings = tf.get_variable(name=position_embedding_name,shape=[max_position_embeddings, width],initializer=create_initializer(initializer_range))position_embeddings = tf.slice(full_position_embeddings, [0, 0],[seq_length, -1])num_dims = len(output.shape.as_list())position_broadcast_shape = []for _ in range(num_dims - 2):position_broadcast_shape.append(1)position_broadcast_shape.extend([seq_length, width])position_embeddings = tf.reshape(position_embeddings,position_broadcast_shape)output += position_embeddingsoutput = layer_norm_and_dropout(output, dropout_prob)return output

主要是信息添加，可以将word的位置和word对应的token type等信息添加到词向量里面，并且layer正则化和dropout之后返回

4、构造attention mask

def create_attention_mask_from_input_mask(from_tensor, to_mask):from_shape = get_shape_list(from_tensor, expected_rank=[2, 3])batch_size = from_shape[0]from_seq_length = from_shape[1]to_shape = get_shape_list(to_mask, expected_rank=2)to_seq_length = to_shape[1]to_mask = tf.cast(tf.reshape(to_mask, [batch_size, 1, to_seq_length]), tf.float32)broadcast_ones = tf.ones(shape=[batch_size, from_seq_length, 1], dtype=tf.float32)mask = broadcast_ones * to_maskreturn mask

将shape为[batch_size, to_seq_length]的2D mask转换为一个shape 为[batch_size, from_seq_length, to_seq_length] 的3D mask用于attention当中。

5、attention layer

def attention_layer(from_tensor,to_tensor,attention_mask=None,num_attention_heads=1,size_per_head=512,query_act=None,key_act=None,value_act=None,attention_probs_dropout_prob=0.0,initializer_range=0.02,do_return_2d_tensor=False,batch_size=None,from_seq_length=None,to_seq_length=None):def transpose_for_scores(input_tensor, batch_size, num_attention_heads,seq_length, width):output_tensor = tf.reshape(input_tensor, [batch_size, seq_length, num_attention_heads, width])output_tensor = tf.transpose(output_tensor, [0, 2, 1, 3])return output_tensorfrom_shape = get_shape_list(from_tensor, expected_rank=[2, 3])to_shape = get_shape_list(to_tensor, expected_rank=[2, 3])if len(from_shape) != len(to_shape):raise ValueError("The rank of `from_tensor` must match the rank of `to_tensor`.")if len(from_shape) == 3:batch_size = from_shape[0]from_seq_length = from_shape[1]to_seq_length = to_shape[1]elif len(from_shape) == 2:if (batch_size is None or from_seq_length is None or to_seq_length is None):raise ValueError("When passing in rank 2 tensors to attention_layer, the values ""for `batch_size`, `from_seq_length`, and `to_seq_length` ""must all be specified.")# Scalar dimensions referenced here:#   B = batch size (number of sequences)#   F = `from_tensor` sequence length#   T = `to_tensor` sequence length#   N = `num_attention_heads`#   H = `size_per_head`from_tensor_2d = reshape_to_matrix(from_tensor)to_tensor_2d = reshape_to_matrix(to_tensor)# `query_layer` = [B*F, N*H]query_layer = tf.layers.dense(from_tensor_2d,num_attention_heads * size_per_head,activation=query_act,name="query",kernel_initializer=create_initializer(initializer_range))# `key_layer` = [B*T, N*H]key_layer = tf.layers.dense(to_tensor_2d,num_attention_heads * size_per_head,activation=key_act,name="key",kernel_initializer=create_initializer(initializer_range))# `value_layer` = [B*T, N*H]value_layer = tf.layers.dense(to_tensor_2d,num_attention_heads * size_per_head,activation=value_act,name="value",kernel_initializer=create_initializer(initializer_range))# `query_layer` = [B, N, F, H]query_layer = transpose_for_scores(query_layer, batch_size,num_attention_heads, from_seq_length,size_per_head)# `key_layer` = [B, N, T, H]key_layer = transpose_for_scores(key_layer, batch_size, num_attention_heads,to_seq_length, size_per_head)attention_scores = tf.matmul(query_layer, key_layer, transpose_b=True)attention_scores = tf.multiply(attention_scores,1.0 / math.sqrt(float(size_per_head)))if attention_mask is not None:# `attention_mask` = [B, 1, F, T]attention_mask = tf.expand_dims(attention_mask, axis=[1])adder = (1.0 - tf.cast(attention_mask, tf.float32)) * -10000.0attention_scores += adderattention_probs = tf.nn.softmax(attention_scores)attention_probs = dropout(attention_probs, attention_probs_dropout_prob)# `value_layer` = [B, T, N, H]value_layer = tf.reshape(value_layer,[batch_size, to_seq_length, num_attention_heads, size_per_head])# `value_layer` = [B, N, T, H]value_layer = tf.transpose(value_layer, [0, 2, 1, 3])# `context_layer` = [B, N, F, H]context_layer = tf.matmul(attention_probs, value_layer)# `context_layer` = [B, F, N, H]context_layer = tf.transpose(context_layer, [0, 2, 1, 3])if do_return_2d_tensor:# `context_layer` = [B*F, N*V]context_layer = tf.reshape(context_layer,[batch_size * from_seq_length, num_attention_heads * size_per_head])else:# `context_layer` = [B, F, N*V]context_layer = tf.reshape(context_layer,[batch_size, from_seq_length, num_attention_heads * size_per_head])return context_layer

整个网络的重头戏来了！tansformer的主要内容都在这里面，输入的from_tensor当作query，to_tensor当作key和value。当self attention的时候from_tensor和to_tensor是同一个值。

（1）函数一开始对输入的shape进行校验，获取batch_size、from_seq_length 、to_seq_length 。输入如果是3D张量则转化成2D矩阵(以输入为word_embedding为例[batch_size, seq_lenth, hidden_size] -> [batch_size*seq_lenth, hidden_size])

（2）通过全连接线性投影生成query_layer、key_layer 、value_layer，输出的第二个维度变成num_attention_heads * size_per_head（整个模型默认hidden_size=num_attention_heads * size_per_head）。然后通过transpose_for_scores转换成多头。

（3）根据公式计算attention_probs（attention score）：

Attention Score计算公式

如果attention_mask is not None，对mask的部分加上一个很大的负数，这样softmax之后相应的概率值接近为0，再dropout。

（4）最后再将value和attention_probs相乘，返回3D张量或者2D矩阵

总结：

同学们可以将这段代码与网络结构图对照起来看：

Attention Layer

该函数相比其他版本的的transformer很多地方都有简化，有以下四点：

（1）缺少scale的操作；

（2）没有Causality mask，个人猜测主要是bert没有decoder的操作，所以对角矩阵mask是不需要的，从另一方面来说正好体现了双向transformer的特点；

（3）没有query mask。跟（2）理由类似，encoder都是self attention，query和key相同所以只需要一次key mask就够了

（4）没有query的Residual层和normalize

6、Transformer

def transformer_model(input_tensor,attention_mask=None,hidden_size=768,num_hidden_layers=12,num_attention_heads=12,intermediate_size=3072,intermediate_act_fn=gelu,hidden_dropout_prob=0.1,attention_probs_dropout_prob=0.1,initializer_range=0.02,do_return_all_layers=False):if hidden_size % num_attention_heads != 0:raise ValueError("The hidden size (%d) is not a multiple of the number of attention ""heads (%d)" % (hidden_size, num_attention_heads))attention_head_size = int(hidden_size / num_attention_heads)input_shape = get_shape_list(input_tensor, expected_rank=3)batch_size = input_shape[0]seq_length = input_shape[1]input_width = input_shape[2]if input_width != hidden_size:raise ValueError("The width of the input tensor (%d) != hidden size (%d)" %(input_width, hidden_size))prev_output = reshape_to_matrix(input_tensor)all_layer_outputs = []for layer_idx in range(num_hidden_layers):with tf.variable_scope("layer_%d" % layer_idx):layer_input = prev_outputwith tf.variable_scope("attention"):attention_heads = []with tf.variable_scope("self"):attention_head = attention_layer(from_tensor=layer_input,to_tensor=layer_input,attention_mask=attention_mask,num_attention_heads=num_attention_heads,size_per_head=attention_head_size,attention_probs_dropout_prob=attention_probs_dropout_prob,initializer_range=initializer_range,do_return_2d_tensor=True,batch_size=batch_size,from_seq_length=seq_length,to_seq_length=seq_length)attention_heads.append(attention_head)attention_output = Noneif len(attention_heads) == 1:attention_output = attention_heads[0]else:attention_output = tf.concat(attention_heads, axis=-1)with tf.variable_scope("output"):attention_output = tf.layers.dense(attention_output,hidden_size,kernel_initializer=create_initializer(initializer_range))attention_output = dropout(attention_output, hidden_dropout_prob)attention_output = layer_norm(attention_output + layer_input)with tf.variable_scope("intermediate"):intermediate_output = tf.layers.dense(attention_output,intermediate_size,activation=intermediate_act_fn,kernel_initializer=create_initializer(initializer_range))with tf.variable_scope("output"):layer_output = tf.layers.dense(intermediate_output,hidden_size,kernel_initializer=create_initializer(initializer_range))layer_output = dropout(layer_output, hidden_dropout_prob)layer_output = layer_norm(layer_output + attention_output)prev_output = layer_outputall_layer_outputs.append(layer_output)if do_return_all_layers:final_outputs = []for layer_output in all_layer_outputs:final_output = reshape_from_matrix(layer_output, input_shape)final_outputs.append(final_output)return final_outputselse:final_output = reshape_from_matrix(prev_output, input_shape)return final_output

transformer是对attention的利用，分以下几步：

（1）计算attention_head_size，attention_head_size = int(hidden_size / num_attention_heads)即将隐层的输出等分给各个attention头。然后将input_tensor转换成2D矩阵；

（2）对input_tensor进行多头attention操作，再做：线性投影——dropout——layer norm——intermediate线性投影——线性投影——dropout——attention_output的residual——layer norm

其中intermediate线性投影的hidden_size可以自行指定，其他层的线性投影hidden_size需要统一，目的是为了对齐。

（3）如此循环计算若干次，且保存每一次的输出，最后返回所有层的输出或者最后一层的输出。

总结：

进一步证实该函数transformer只存在encoder，而不存在decoder操作，所以所有层的多头attention操作都是基于self encoder的。对应论文红框的部分：

The Transformer - model architecture

7、BertModel

class BertModel(object):def __init__(self,config,is_training,input_ids,input_mask=None,token_type_ids=None,use_one_hot_embeddings=True,scope=None):config = copy.deepcopy(config)if not is_training:config.hidden_dropout_prob = 0.0config.attention_probs_dropout_prob = 0.0input_shape = get_shape_list(input_ids, expected_rank=2)batch_size = input_shape[0]seq_length = input_shape[1]if input_mask is None:input_mask = tf.ones(shape=[batch_size, seq_length], dtype=tf.int32)if token_type_ids is None:token_type_ids = tf.zeros(shape=[batch_size, seq_length], dtype=tf.int32)with tf.variable_scope(scope, default_name="bert"):with tf.variable_scope("embeddings"):(self.embedding_output, self.embedding_table) = embedding_lookup(input_ids=input_ids,vocab_size=config.vocab_size,embedding_size=config.hidden_size,initializer_range=config.initializer_range,word_embedding_name="word_embeddings",use_one_hot_embeddings=use_one_hot_embeddings)self.embedding_output = embedding_postprocessor(input_tensor=self.embedding_output,use_token_type=True,token_type_ids=token_type_ids,token_type_vocab_size=config.type_vocab_size,token_type_embedding_name="token_type_embeddings",use_position_embeddings=True,position_embedding_name="position_embeddings",initializer_range=config.initializer_range,max_position_embeddings=config.max_position_embeddings,dropout_prob=config.hidden_dropout_prob)with tf.variable_scope("encoder"):attention_mask = create_attention_mask_from_input_mask(input_ids, input_mask)self.all_encoder_layers = transformer_model(input_tensor=self.embedding_output,attention_mask=attention_mask,hidden_size=config.hidden_size,num_hidden_layers=config.num_hidden_layers,num_attention_heads=config.num_attention_heads,intermediate_size=config.intermediate_size,intermediate_act_fn=get_activation(config.hidden_act),hidden_dropout_prob=config.hidden_dropout_prob,attention_probs_dropout_prob=config.attention_probs_dropout_prob,initializer_range=config.initializer_range,do_return_all_layers=True)self.sequence_output = self.all_encoder_layers[-1]with tf.variable_scope("pooler"):first_token_tensor = tf.squeeze(self.sequence_output[:, 0:1, :], axis=1)self.pooled_output = tf.layers.dense(first_token_tensor,config.hidden_size,activation=tf.tanh,kernel_initializer=create_initializer(config.initializer_range))

终于到模型入口了。

(1）设置各种参数，如果input_mask为None的话，就指定所有input_mask值为1，即不进行过滤；如果token_type_ids是None的话，就指定所有token_type_ids值为0；

(2）对输入的input_ids进行embedding操作，再embedding_postprocessor操作，前面我们说了。主要是加入位置和token_type信息到词向量里面；

(3）转换attention_mask 后，通过调用transformer_model进行encoder操作；

(4）获取最后一层的输出sequence_output和pooled_output，pooled_output是取sequence_output的第一个切片然后线性投影获得（可以用于分类问题）

8、总结：

（1）bert主要流程是先embedding（包括位置和token_type的embedding），然后调用transformer得到输出结果，其中embedding、embedding_table、所有transformer层输出、最后transformer层输出以及pooled_output都可以获得，用于迁移学习的fine-tune和预测任务；

（2）bert对于transformer的使用仅限于encoder，没有decoder的过程。这是因为模型存粹是为了预训练服务，而预训练是通过语言模型，不同于NLP其他特定任务。在做迁移学习时可以自行添加；

（3）正因为没有decoder的操作，所以在attention函数里面也相应地减少了很多不必要的功能。

其他非主要函数这里不做过多介绍，感兴趣的同学可以去看源码。

下一篇文章我们将继续学习bert源码的其他模块，包括训练、预测以及输入输出等相关功能。

本文上一篇系列

Bert系列（一）——demo运行
Bert系列（三）——源码解读之Pre-train
Bert系列（四）——源码解读之Fine-tune
Bert系列（五）——中文分词实践 F1 97.8%(附代码)

Reference

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

2.https://github.com/Kyubyong/transformer

3.Attention Is All You Need

4.BERT: Pre-training of Deep Bidirectional Transformers for
Language Understanding

作者：西溪雷神
链接：https://www.jianshu.com/p/d7ce41b58801
来源：简书
简书著作权归作者所有，任何形式的转载都请联系作者获得授权并注明出处。