🗃️ Use specified data_collator in RLOOTrainer and PPOTrainer

[bartoszzuk]

What does this PR do?

• Use the specified data_collator instead of using hard-coded DataCollatorWithPadding
• When no data_collator is specified use DataCollatorWithPadding (previous behavior)

Fixes #2354

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.

[kashif]

thanks @bartoszzuk perhaps it's better to set the self.data_collator to the default one if it is none and then use self.data_collator in the data loaders?

[kashif]

you might need to run make precommit in the root of the TRL to fix styling

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.