[Flax][WIP] added Flax Pegasus Models
# What does this PR do?

## 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?
@patil-suraj @patrickvonplaten 
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 @bhadreshpsavani thanks for the PR, let us know if you want to continue working on this :) <|||||>Sure @patil-suraj,
I want to continue on it but I will need some help from you I think!
Shall I contact you slack If I need any guidance from you?
I will update this PR today with latest changes!<|||||>Sounds good, happy to help :)<|||||>Given that we have a cookie-cutter example now, it might be worth actually starting from scratch using the cookie-cutter that is based on BART - will probably be more efficient<|||||>Hi @patrickvonplaten 
I will start from scratch! That would be better.
I create another PR, that will be fine right?<|||||>This one is being handled by above mentioned PR