[Draft] Add eval_data_collator arg

[pdufour]

What does this PR do?

Note: not ready for review until huggingface/transformers#32346 lands.

This adds support for the new eval_data_collator arg which was added in the above PR.

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,
  Pull Request section?
• Was this discussed/approved via a GitHub issue? 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.
• Did you write any new necessary tests?

Who can review?

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.

[tcz]

How would you make an eval_data_collator from DataCollatorForCompletionOnlyLM?