Consolidate slow tests into main test files

[ishitab02]

Summary

What does this PR do?

This PR consolidates slow test cases from the tests/slow/ directory into their respective main test files to improve test organization and maintainability.

Fixes #4401

Changes

• Moved test cases from tests/slow/test_dpo_slow.py to tests/test_dpo_trainer.py
• Moved test cases from tests/slow/test_grpo_slow.py to tests/test_grpo_trainer.py
• Moved test cases from tests/slow/test_sft_slow.py to tests/test_sft_trainer.py
• Added missing imports (gc, BitsAndBytesConfig) that were causing linting errors
• Removed obsolete tests/slow/ test files

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 make sure to update the documentation with your changes?
• Did you write any new necessary tests?

[qgallouedec]

lgtm, @albertvillanova is it ok for you as well?
In my opinion, most of these slow test could be removed, and are mostly redundant with non-slow test, but this is out of the scope of this PR.

[albertvillanova]

It looks good to me. I just fixed the conflicts with main.

[HuggingFaceDocBuilderDev]

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