🗃️ 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

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• [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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• [ ] Did you write any new necessary tests?

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[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]

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