

TITLE
Modify BERT encoder layers?

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Hello I would like to modify the encoder layers of the BERT model, to insert FC and ReLu layers.

This idea allows you to reproduce the use of [Squeeze-and-Excitation Networks](https://arxiv.org/abs/1709.01507)

How to use an nn.module class to handle encoder outputs?

Example:

```
import torch.nn as nn
from transformers import BertModel

class CustomBERTModel(nn.Module):
    def __init__(self):
          super(CustomBERTModel, self).__init__()
          self.bert = BertModel.from_pretrained("bert-base-uncased")
          # add your additional layers here, for example a dropout layer followed by a linear classification head
          self.dropout = nn.Dropout(0.3)
          self.out = nn.Linear(768, 2)

    def forward(self, ids, mask, token_type_ids):
          sequence_output, pooled_output = self.bert(
               ids, 
               attention_mask=mask,
               token_type_ids=token_type_ids
          )

          # we apply dropout to the sequence output, tensor has shape (batch_size, sequence_length, 768)
          sequence_output = self.dropout(sequence_output)
    
          # next, we apply the linear layer. The linear layer (which applies a linear transformation)
          # takes as input the hidden states of all tokens (so seq_len times a vector of size 768, each corresponding to
          # a single token in the input sequence) and outputs 2 numbers (scores, or logits) for every token
          # so the logits are of shape (batch_size, sequence_length, 2)
          logits = self.out(sequence_output)

          return logits
```