[Flax] Add FlaxMBart
# What does this PR do?
This PR adds flax implementation of MBart.


## 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?

@patrickvonplaten  @patil-suraj
@patil-suraj Thank you a lot for your suggestions. I fixed the order of attention and normalization layers and some other minor bugs. Also added some additional copy statements.

I also changed the `shift_tokens_right` method as this one looks to be different for the MBart models as they don't have a single `decoder_start_token_id` in contrast to other Bart-like models. => This difference of having no `decoder_start_token_id`, however, currently leads to some issues within the `generate` method. (I'll try to have a look what can be done here)
<|||||>@stancld I pushed a couple of commits to add the `layer_norm` in encoder and decoder. Now, all slow tests are passing.
@patrickvonplaten  could you please take a final look?