📏 torch_dype to dtype everywhere

[sergiopaniego]

What does this PR do?

Following this PR that deprecates torch_dtype in favor of dtype, this PR introduces the same change to trl.

Legacy support for torch_dtype is still provided in to model_config.py but deprecated in a future version.

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?
• Did you write any new necessary tests?

Who can review?

@qgallouedec @kashif

[HuggingFaceDocBuilderDev]

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[qgallouedec]

Thanks!
This requires bumping the min transformers version to 4.56

[qgallouedec]

You can safely ignore the failing CI, see huggingface/transformers#40668

[qgallouedec]

LGTM!