PyTorch 1.2 has released API 'torch.nn.Transformer'，so it's better to modify the source code with the official  python API
## 🚀 Feature

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## Motivation

It's better to Modify modeling_bert.py with official API 'torch.nn.Transformer' of PyTorch 1.2

## Additional context

https://pytorch.org/docs/stable/nn.html?highlight=transformer#torch.nn.Transformer

This has been suggested a while back when this was first introduced (we're at 1.4 now). This is possibly impractical to do since it is likely that many people are still on 1.0<=x<1.2. <|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
