Update trainer.py: add multiprocessing_context for mps devices

[AmitMY]

What does this PR do?

I have encountered an error during training on an M1 mac, similar to: https://github.com/pytorch/pytorch/issues/87688

I have implemented the suggested fix, and it works.

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, Pull Request section?
• [ ] Was this discussed/approved via a Github issue or the forum? 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, and here are tips on formatting docstrings.
• [ ] Did you write any new necessary tests?

Who can review?

• trainer: @zach-huggingface, @SunMarc and @qgallouedec