What does this PR do?

https://github.com/huggingface/transformers/pull/33514 this PR adds group_by_length support for evaluation. But, this part uses self.eval_dataset instead of eval_dataset. So, if eval_dataset is dictionary it fails.

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, 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?

• trainer: @muellerzr and @SunMarc