Fix missing metrics with multiple eval datasets
Fixes #22530
[Issue](https://github.com/huggingface/transformers/issues/22530)
**tl;dr** - `Trainer` only keeps the last metric when using multiple eval datasets. This PR fixes that by merging metrics from all eval datasets into one dict.

## 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).
- [x] Did you write any new necessary tests?

@sgugger 

_The documentation is not available anymore as the PR was closed or merged._<|||||>It seems like the tests failed on an irrelevant "File not found error" in tokenizers, but I can not rerun the tests. 
@sgugger would you kindly trigger them again?<|||||>:call_me_hand: 