DPO support remove_unused_columns

[qgallouedec]

What does this PR do?

Currently,

1. removing unused column isn't supported
2. when not explicitly passing --no_remove_unused_columns, the following warning appears:

$ accelerate launch examples/scripts/dpo.py --output_dir Qwen2-0.5B-DPO --dataset_name trl-lib/lm-human-preferences-sentiment --model_name_or_path Qwen/Qwen2-0.5B-Instruct --logging_steps 10 --report_to none
/fsx/qgallouedec/trl/trl/trainer/dpo_trainer.py:720: UserWarning: When using DPODataCollatorWithPadding, you should set `remove_unused_columns=False` in your TrainingArguments. We have set it for you, but you should do it yourself in the future.
  warnings.warn(

This PR adds support for removing unused columns, which resolves the warning.

Does it remove unused columns?

	Before	After
--remove_unused_columns (default)	No	Yes
--no_remove_unused_columns	No	No

Does the training run successfully?

	Before	After
--remove_unused_columns (default)	Yes, but raises a warning	Yes
--no_remove_unused_columns	Yes	Yes

Context

Trying to split #2209 into smaller pieces

Before submitting

• 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?

Who can review?

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[qgallouedec]

For any warning, it should be possible for the user to take an action that removes it. In this case, it's not possible. Therefore, I suggest deleting it.

[HuggingFaceDocBuilderDev]

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