Adding missing type hints for mBART model (PyTorch)
PyTorch Implementation model added with missing type hints

# What does this PR do?

Added type hints for mBART PyTorch as described in https://github.com/huggingface/transformers/issues/16059


## 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.
- [ ] 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).
- [ ] Did you write any new necessary tests?


## Who can review?

@Rocketknight1 @gante 
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.

_The documentation is not available anymore as the PR was closed or merged._<|||||>Hi, this looks good, and I'm sorry for the delay! I'm guessing the failing tests are due to issues we fixed recently, and would go away if you rebased the PR onto the most recent `huggingface/transformers/main` and force-pushed. If you're not sure how to do that, just ping me here or on Discord and I'll give you instructions!