Fix: Seq2SeqTrainingArgs overriding to_dict for GenerationConfig json support
# What does this PR do?

`Seq2SeqTrainingArguments` override the `to_dict()` method from `TrainingArguments`.

This is a fix to #22831 (solution 2), solving an error that happened when saving to json a `Seq2SeqTrainingArguments` object with a `GenerationConfig` attribute.


## 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/)? #22831
- [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?

@sgugger @gante 

_The documentation is not available anymore as the PR was closed or merged._<|||||>Just updated<|||||>Thanks for adding this @Natooz 💛 