Update no_trainer script for summarization
# What does this PR do?

Update `run_summarization_no_trainer.py` in the example to include `accelerator.gather_metrics`

Fixes a part of #18437

## 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?
@muellerzr , @sgugger 
Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

_The documentation is not available anymore as the PR was closed or merged._<|||||>> Thanks a lot for your PR! You also need to remove the initialization of `samples_seen` since that variable is not used anymore. This will fix the quality check.

Done; thanks for pointing it out.