bert在预训练时的两个下游任务详解

众所周知，bert预训练有加入了两个下游任务进行训练，分别是next sentence prediction和mask prediction。

next sentence prediction：输入[CLS]a[SEP]b[SEP]，预测b是否为a的下一句，即二分类问题；
mask prediction：输入[CLS]我 mask 中 mask 天安门[SEP]，预测句子的mask，多分类问题

一直很困惑bert在经过transformer的encoder之后的两个任务是怎么做的。比如：

这两个任务的loss函数是什么？
拿到encoder输出做了什么？

很多人都是这么回答的：

bert的loss就是下一句预测的分类loss和mask词的预测loss
拿encoder输出直接算loss啊

如果你继续问：

那他们的loss都分别怎么算的？

相信很多人都讲不出来。下面就详细在这阐述下。相信你会对bert更了解的。

源码解读：

BertModel类中的forward部分代码如下

encoded_layers表示encoder所有层隐层状态输出；sequence_output表示最后一层。首先经过一个self.pooler变换：

注意看：将sequence_output输入BertPooler这个类之后，取了句子第一个token（first_token_tensor，即CLS）的向量，然后将first_token_tensor输入self.dense。从该类的初始化中可知，self.dense是一个线性层，参数为768 * 768。经过该线性层之后，又过了一个Tanh激活函数。然后得到pooled_output(1 * 768 )。

经过BertModel类之后，返回了两个输出sequence_output(len * 768)和pooled_output(1 * 768 )。

然后经过self.cls，即类BertPreTrainingHeads。

经过BertPreTrainingHeads时，先进过self.predictions。即类BertLMPredictionHead：

在BertLMPredictionHead里面又经过self.transformer和self.decoder

self.transformer中经历了self.dense(768 * 768线性层)，self.transform_act_fn激活函数（根据bert config，这里应该为gelu），self.LayerNorm(归一化处理)

self.decoder中经历了一个线性层（768 * vocab_size），因为
bert_model_embedding_weights共享了词向量参数，因此该线性层为768 * vocab_size。

终于走到底了，那么现在开始往回走：重新给梳理下，从下往上走：
2个输入：
1、sequence_output（len * 768 ）：最后一层隐层状态。依次经过self.transformer(768 * 768线性层) -> self.transform_act_fn(tanh激活函数) -> self.LayerNorm(归一化) -> self.decoder(768 * vocab_size，共享词向量参数)
经过这一系列之后，我们就拿到了prediction_scores（len * vocab_size）然后我们就可以直接算loss了，采用的是CrossEntropyLoss交叉熵函数。

2、pooled_output（1 * 768 ）：CLS隐层状态。依次经过self.seq_relationship（768 * 2线性层）
经过这一系列之后，我们就拿到了seq_relationship_score(1 * 2 )，二分类。然后我们就可以直接算loss了，采用的同样是CrossEntropyLoss交叉熵函数。

最后，loss = 上述两部分的loss之和

因此，这也就解释了，为什么bert的参数中包含了以下几个参数了

bert.pooler.dense.weight
bert.pooler.dense.bias
classifier.weight
classifier.bias

如果不正确之处，请不吝指正，感激不尽，谢谢！

下面是所有bert预训练参数：

bert.embeddings.word_embeddings.weight
bert.embeddings.position_embeddings.weight
bert.embeddings.token_type_embeddings.weight
bert.embeddings.LayerNorm.weight
bert.embeddings.LayerNorm.bias
bert.encoder.layer.0.attention.self.query.weight
bert.encoder.layer.0.attention.self.query.bias
bert.encoder.layer.0.attention.self.key.weight
bert.encoder.layer.0.attention.self.key.bias
bert.encoder.layer.0.attention.self.value.weight
bert.encoder.layer.0.attention.self.value.bias
bert.encoder.layer.0.attention.output.dense.weight
bert.encoder.layer.0.attention.output.dense.bias
bert.encoder.layer.0.attention.output.LayerNorm.weight
bert.encoder.layer.0.attention.output.LayerNorm.bias
bert.encoder.layer.0.intermediate.dense.weight
bert.encoder.layer.0.intermediate.dense.bias
bert.encoder.layer.0.output.dense.weight
bert.encoder.layer.0.output.dense.bias
bert.encoder.layer.0.output.LayerNorm.weight
bert.encoder.layer.0.output.LayerNorm.bias
bert.encoder.layer.1.attention.self.query.weight
bert.encoder.layer.1.attention.self.query.bias
bert.encoder.layer.1.attention.self.key.weight
bert.encoder.layer.1.attention.self.key.bias
bert.encoder.layer.1.attention.self.value.weight
bert.encoder.layer.1.attention.self.value.bias
bert.encoder.layer.1.attention.output.dense.weight
bert.encoder.layer.1.attention.output.dense.bias
bert.encoder.layer.1.attention.output.LayerNorm.weight
bert.encoder.layer.1.attention.output.LayerNorm.bias
bert.encoder.layer.1.intermediate.dense.weight
bert.encoder.layer.1.intermediate.dense.bias
bert.encoder.layer.1.output.dense.weight
bert.encoder.layer.1.output.dense.bias
bert.encoder.layer.1.output.LayerNorm.weight
bert.encoder.layer.1.output.LayerNorm.bias
bert.encoder.layer.2.attention.self.query.weight
bert.encoder.layer.2.attention.self.query.bias
bert.encoder.layer.2.attention.self.key.weight
bert.encoder.layer.2.attention.self.key.bias
bert.encoder.layer.2.attention.self.value.weight
bert.encoder.layer.2.attention.self.value.bias
bert.encoder.layer.2.attention.output.dense.weight
bert.encoder.layer.2.attention.output.dense.bias
bert.encoder.layer.2.attention.output.LayerNorm.weight
bert.encoder.layer.2.attention.output.LayerNorm.bias
bert.encoder.layer.2.intermediate.dense.weight
bert.encoder.layer.2.intermediate.dense.bias
bert.encoder.layer.2.output.dense.weight
bert.encoder.layer.2.output.dense.bias
bert.encoder.layer.2.output.LayerNorm.weight
bert.encoder.layer.2.output.LayerNorm.bias
bert.encoder.layer.3.attention.self.query.weight
bert.encoder.layer.3.attention.self.query.bias
bert.encoder.layer.3.attention.self.key.weight
bert.encoder.layer.3.attention.self.key.bias
bert.encoder.layer.3.attention.self.value.weight
bert.encoder.layer.3.attention.self.value.bias
bert.encoder.layer.3.attention.output.dense.weight
bert.encoder.layer.3.attention.output.dense.bias
bert.encoder.layer.3.attention.output.LayerNorm.weight
bert.encoder.layer.3.attention.output.LayerNorm.bias
bert.encoder.layer.3.intermediate.dense.weight
bert.encoder.layer.3.intermediate.dense.bias
bert.encoder.layer.3.output.dense.weight
bert.encoder.layer.3.output.dense.bias
bert.encoder.layer.3.output.LayerNorm.weight
bert.encoder.layer.3.output.LayerNorm.bias
bert.encoder.layer.4.attention.self.query.weight
bert.encoder.layer.4.attention.self.query.bias
bert.encoder.layer.4.attention.self.key.weight
bert.encoder.layer.4.attention.self.key.bias
bert.encoder.layer.4.attention.self.value.weight
bert.encoder.layer.4.attention.self.value.bias
bert.encoder.layer.4.attention.output.dense.weight
bert.encoder.layer.4.attention.output.dense.bias
bert.encoder.layer.4.attention.output.LayerNorm.weight
bert.encoder.layer.4.attention.output.LayerNorm.bias
bert.encoder.layer.4.intermediate.dense.weight
bert.encoder.layer.4.intermediate.dense.bias
bert.encoder.layer.4.output.dense.weight
bert.encoder.layer.4.output.dense.bias
bert.encoder.layer.4.output.LayerNorm.weight
bert.encoder.layer.4.output.LayerNorm.bias
bert.encoder.layer.5.attention.self.query.weight
bert.encoder.layer.5.attention.self.query.bias
bert.encoder.layer.5.attention.self.key.weight
bert.encoder.layer.5.attention.self.key.bias
bert.encoder.layer.5.attention.self.value.weight
bert.encoder.layer.5.attention.self.value.bias
bert.encoder.layer.5.attention.output.dense.weight
bert.encoder.layer.5.attention.output.dense.bias
bert.encoder.layer.5.attention.output.LayerNorm.weight
bert.encoder.layer.5.attention.output.LayerNorm.bias
bert.encoder.layer.5.intermediate.dense.weight
bert.encoder.layer.5.intermediate.dense.bias
bert.encoder.layer.5.output.dense.weight
bert.encoder.layer.5.output.dense.bias
bert.encoder.layer.5.output.LayerNorm.weight
bert.encoder.layer.5.output.LayerNorm.bias
bert.encoder.layer.6.attention.self.query.weight
bert.encoder.layer.6.attention.self.query.bias
bert.encoder.layer.6.attention.self.key.weight
bert.encoder.layer.6.attention.self.key.bias
bert.encoder.layer.6.attention.self.value.weight
bert.encoder.layer.6.attention.self.value.bias
bert.encoder.layer.6.attention.output.dense.weight
bert.encoder.layer.6.attention.output.dense.bias
bert.encoder.layer.6.attention.output.LayerNorm.weight
bert.encoder.layer.6.attention.output.LayerNorm.bias
bert.encoder.layer.6.intermediate.dense.weight
bert.encoder.layer.6.intermediate.dense.bias
bert.encoder.layer.6.output.dense.weight
bert.encoder.layer.6.output.dense.bias
bert.encoder.layer.6.output.LayerNorm.weight
bert.encoder.layer.6.output.LayerNorm.bias
bert.encoder.layer.7.attention.self.query.weight
bert.encoder.layer.7.attention.self.query.bias
bert.encoder.layer.7.attention.self.key.weight
bert.encoder.layer.7.attention.self.key.bias
bert.encoder.layer.7.attention.self.value.weight
bert.encoder.layer.7.attention.self.value.bias
bert.encoder.layer.7.attention.output.dense.weight
bert.encoder.layer.7.attention.output.dense.bias
bert.encoder.layer.7.attention.output.LayerNorm.weight
bert.encoder.layer.7.attention.output.LayerNorm.bias
bert.encoder.layer.7.intermediate.dense.weight
bert.encoder.layer.7.intermediate.dense.bias
bert.encoder.layer.7.output.dense.weight
bert.encoder.layer.7.output.dense.bias
bert.encoder.layer.7.output.LayerNorm.weight
bert.encoder.layer.7.output.LayerNorm.bias
bert.encoder.layer.8.attention.self.query.weight
bert.encoder.layer.8.attention.self.query.bias
bert.encoder.layer.8.attention.self.key.weight
bert.encoder.layer.8.attention.self.key.bias
bert.encoder.layer.8.attention.self.value.weight
bert.encoder.layer.8.attention.self.value.bias
bert.encoder.layer.8.attention.output.dense.weight
bert.encoder.layer.8.attention.output.dense.bias
bert.encoder.layer.8.attention.output.LayerNorm.weight
bert.encoder.layer.8.attention.output.LayerNorm.bias
bert.encoder.layer.8.intermediate.dense.weight
bert.encoder.layer.8.intermediate.dense.bias
bert.encoder.layer.8.output.dense.weight
bert.encoder.layer.8.output.dense.bias
bert.encoder.layer.8.output.LayerNorm.weight
bert.encoder.layer.8.output.LayerNorm.bias
bert.encoder.layer.9.attention.self.query.weight
bert.encoder.layer.9.attention.self.query.bias
bert.encoder.layer.9.attention.self.key.weight
bert.encoder.layer.9.attention.self.key.bias
bert.encoder.layer.9.attention.self.value.weight
bert.encoder.layer.9.attention.self.value.bias
bert.encoder.layer.9.attention.output.dense.weight
bert.encoder.layer.9.attention.output.dense.bias
bert.encoder.layer.9.attention.output.LayerNorm.weight
bert.encoder.layer.9.attention.output.LayerNorm.bias
bert.encoder.layer.9.intermediate.dense.weight
bert.encoder.layer.9.intermediate.dense.bias
bert.encoder.layer.9.output.dense.weight
bert.encoder.layer.9.output.dense.bias
bert.encoder.layer.9.output.LayerNorm.weight
bert.encoder.layer.9.output.LayerNorm.bias
bert.encoder.layer.10.attention.self.query.weight
bert.encoder.layer.10.attention.self.query.bias
bert.encoder.layer.10.attention.self.key.weight
bert.encoder.layer.10.attention.self.key.bias
bert.encoder.layer.10.attention.self.value.weight
bert.encoder.layer.10.attention.self.value.bias
bert.encoder.layer.10.attention.output.dense.weight
bert.encoder.layer.10.attention.output.dense.bias
bert.encoder.layer.10.attention.output.LayerNorm.weight
bert.encoder.layer.10.attention.output.LayerNorm.bias
bert.encoder.layer.10.intermediate.dense.weight
bert.encoder.layer.10.intermediate.dense.bias
bert.encoder.layer.10.output.dense.weight
bert.encoder.layer.10.output.dense.bias
bert.encoder.layer.10.output.LayerNorm.weight
bert.encoder.layer.10.output.LayerNorm.bias
bert.encoder.layer.11.attention.self.query.weight
bert.encoder.layer.11.attention.self.query.bias
bert.encoder.layer.11.attention.self.key.weight
bert.encoder.layer.11.attention.self.key.bias
bert.encoder.layer.11.attention.self.value.weight
bert.encoder.layer.11.attention.self.value.bias
bert.encoder.layer.11.attention.output.dense.weight
bert.encoder.layer.11.attention.output.dense.bias
bert.encoder.layer.11.attention.output.LayerNorm.weight
bert.encoder.layer.11.attention.output.LayerNorm.bias
bert.encoder.layer.11.intermediate.dense.weight
bert.encoder.layer.11.intermediate.dense.bias
bert.encoder.layer.11.output.dense.weight
bert.encoder.layer.11.output.dense.bias
bert.encoder.layer.11.output.LayerNorm.weight
bert.encoder.layer.11.output.LayerNorm.bias
bert.pooler.dense.weight
bert.pooler.dense.bias
cls.predictions.bias
cls.predictions.transform.dense.weight
cls.predictions.transform.dense.bias
cls.predictions.transform.LayerNorm.gamma
cls.predictions.transform.LayerNorm.beta
cls.predictions.decoder.weight
cls.seq_relationship.weight
cls.seq_relationship.bias