Implement head_mask for Flax BERT and other models copied from BERT
# What does this PR do?

This PR implements attention head masking for Flax BERT models, and other models copied from BERT, i.e. Roberta, BigBird, Electra. This PR thus narrows the gap between Flax and other implementations.


## Before submitting
- [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. Feel free to tag
members/contributors who may be interested in your PR.
@LysandreJik 


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Hi @patil-suraj, thank you a lot for the review. I commented on the issue with BigBird head-masking. As I mentioned, I tried to follow the current `PyTorch` implementation, and I'm definitely willing to have a look at handling head masking for different attention mechanisms.<|||||>I forgot to write this earlier, do you think you could also update the flax templates, the templates test are failing, 
If not let me know, I will take care of it , thanks :) <|||||>@patil-suraj Oh, good point. I'll update the template :]