changes for mbart_causal_lm
# What does this PR do?

This PR add FlaxMBartForCausalLM which was previously part of #19831, as discussed in [here](https://github.com/huggingface/transformers/issues/19897#issuecomment-1294648919), I am raising a separate PR for it.

Reason I want to add this model is, that it is a prerequisite to Donut model.

## 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/main/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/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [x] Did you write any new necessary tests?


## Who can review?
@sanchit-gandhi 
@sanchit-gandhi please have a look when you have some time to spare, for some reason test did not ran. can you check the same and please re-initialize the test pipeline<|||||>Might need to fix style according to https://huggingface.co/docs/transformers/pr_checks#code-and-documentation-style<|||||>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/main/CONTRIBUTING.md) are likely to be ignored.