[`MBart`] Add `accelerate` support for MBart
# What does this PR do?

Partially fixes: #22305 

This PR adds  `accelerate` support for `MBart` models.
To run the `accelerate` tests: 

```bash
RUN_SLOW=1 pytest -m accelerate_tests tests/models/mbart/test_modeling_mbart.py
```
A fix similar to https://github.com/huggingface/transformers/pull/19927 needs to be applied in order for the tests to pass

cc @amyeroberts 
_The documentation is not available anymore as the PR was closed or merged._