Squeeze and Excitation Network
# What does this PR do?
This PR implements an optional Squeeze and Excitation Block in Bert and the copied modules (RoBerta, Electra, splinter and layoutlm) in pytorch. 
Fixes #11998 
Additional tests have been added to the corresponding test scripts and the docs updated.

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?

## Who can review?
Anyone in the community is free to review the PR once the tests have passed. 
@LysandreJik 
Hi,

Thanks for your PR! However, I don't think that we want to add this block to files of other models. It's more appropriate to add a new SesameBERT model (if pretrained weights are available), or add it under the `research_projects` directory.

cc @LysandreJik <|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.<|||||>Hi @AdityaDas-IITM, are you interested in working on this PR (making it a research project instead)?<|||||>Hey @NielsRogge, Yes I'll get started on it soon<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md) are likely to be ignored.