Add logits_processor parameter, used by `generate`, to `Seq2SeqTrainer` methods `evaluate` and `predict`
# What does this PR do?

Following the discussion in #17748, this PR adds the `logits_processor` param to Seq2SeqTrainer `predict` and `evaluate` methods, for easy extensibility.


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@gante @sgugger
_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger What are your thoughts about adding to the `test_finetune_bert2bert` a call to `trainer.evaluate()` and `trainer.predict(eval_dataset)`?<|||||>Sure, we can add this. But is it to test any of this functionality? If not, it should go in its own PR.<|||||>It somewhat tests this functionality by calling the methods I changed, but not directly. I can add it later in its own PR.<|||||>Merging this one then, thanks again for your contribution!