make dpo compatible with qwen3vl

[flutist]

What does this PR do?

Fixes # (issue)
make dpo compatible with qwen3vl

Before submitting

• [yes ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• [ yes] Did you read the contributor guideline,
  Pull Request section?
• [no ] Was this discussed/approved via a GitHub issue? Please add a link
  to it if that's the case.
• [yes ] Did you make sure to update the documentation with your changes?
• [ no] 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
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[qgallouedec]

Thanks, can you add a test case using trl-internal-testing/tiny-Qwen3VLForConditionalGeneration in TestDPOVisionTrainer to ensure it works properly?

[flutist]

Thanks, can you add a test case using trl-internal-testing/tiny-Qwen3VLForConditionalGeneration in TestDPOVisionTrainer to ensure it works properly?

Of course, let me implement one.

[flutist]

Thanks, can you add a test case using trl-internal-testing/tiny-Qwen3VLForConditionalGeneration in TestDPOVisionTrainer to ensure it works properly?

I have finished the work, and all the test is passed.

[flutist]

@qgallouedec could you help to do a code review for this request?

[flutist]

One more thing: these three default values ​​are all quite small. During actual training, it's easy to overlook setting these values, which can lead to errors. Do they need to be increased? @qgallouedec

[qgallouedec]

these three default values ​​are all quite small. During actual training, it's easy to overlook setting these values, which can lead to errors. Do they need to be increased? @qgallouedec

max_prompt_length and max_completion_length should be removed as part of #3906

[flutist]

these three default values ​​are all quite small. During actual training, it's easy to overlook setting these values, which can lead to errors. Do they need to be increased? @qgallouedec

max_prompt_length and max_completion_length should be removed as part of #3906

got that, thanks

[flutist]

@qgallouedec
Are there any codes that need further modification?
I need your feedback.
Thanks much.