Fix no trainer summarization script test failure
# What does this PR do?

This PR solves the test failure in the nightly CI for the summarization no trainer script. Also potentially https://github.com/huggingface/transformers/issues/18189 too, just waiting for verification first otherwise a followup PR will be made

`gather_for_metrics` was being called twice instead of once, along with a syntax error, which is not good! :) 

Fixes # (issue)


## 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?
- [ ] 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.
- [ ] 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?

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.

@sgugger 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_20286). All of your documentation changes will be reflected on that endpoint.