Fix forced_bos_token_id not set in generation_config

[Addyk-24]

What does this PR do?

Fixes #41492

Fixes incorrect target language generation during evaluation/validation in run_translation.py for multilingual translation models (mBART , M2M100).

Problem

When fine-tuning multilingual models, forced_bos_token_id was only set in model.config but not in model.generation_config. During evaluation, model.generate() reads from generation_config, causing generation in wrong language and artificially low BLEU scores.Previously would be ~2-5 (wrong language)

Solution

Set forced_bos_token_id in both model.config and model.generation_config.

Results:

• ✅ BLEU score: 29.07
• ✅ This warning appears if you modify model.config directly for generation. Using model.generation_config removes this warning and ensures Transformers v5+ uses the setting correctly.
• ✅ All evaluations complete without errors

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 or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

@zach-huggingface @Cyrilvallez

[Bmingg]

What's worked for me is that I set model.generation_config.decoder_start_token_id to the target language ID of MBart. When I discovered this bug, I think I checked the forced_bos_token_id of the output of MBart, and it should still be the start token s>. In this case, I think what was missing was the target language ID after start token, if I remembered correctly.