[Flax] Add Electra models
# What does this PR do?

- Implement Flax version of Electra model and classes for different downstream tasks: 
  - `FlaxElectraModel`
  - `FlaxElectraForMaskedLM`
  - `FlaxElectraForPreTraining`
  - `FlaxElectraForMultipleChoice`
  - `FlaxElectraForQuestionAnswering`
  - `FlaxElectraForSequenceClassification`
  - `FlaxElectraForTokenClassification`
 
Most of the code taken from FlaxBert code and the Pytorch Electra code, and also adapted code from the original PR from @chris-tng  (credit where it's due since he started this in #9172 ).

Running the tests (including the slow ones) works, and I already tested in my downstream task and seems to be working


## 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?
- [ ] 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.

Tagging @patrickvonplaten @sgugger @chris-tng , feel free to tag other people


I think I address most comments (the only thing missing is removing the `from_pt` when the flax checkpoint gets upload)! Thanks for the swift feedback<|||||>Hey @CoderPat,

We've merged a last design extension for the Flax design today, [here](https://github.com/huggingface/transformers/commit/f748bd424213ca8e76e6ad9ffe2beece2ff2655e) -> could you merge master into your PR one last time and adapt the code to add those extensions (example docstring + model outputs + all_attentions + all_hidden_states) - super sorry for the merge conflict again, but this will be the last one!