[Flax][WIP] Addition of Flax-Wav2Vec Model
# What does this PR do?
This PR is for the addition of Wav2Vec Model 

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [ ] 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.
- [ ] 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).
- [ ] Did you write any new necessary tests?


## Who can review?
@patil-suraj
Cool PR! For next steps, we should write the missing classes and remove everything which is related to:

```
feat_extract_norm="group"
do_stable_layer_norm=True
```

(This config parameters are only used for https://huggingface.co/facebook/wav2vec2-base-960h which is the oldest of the wav2vec2 models)

Also, it would be very important to add tests in `modeling_flax_wav2vec2.py` <|||||>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.<|||||>Closing this in favor of https://github.com/huggingface/transformers/pull/12271