Add decorator to flaky accelerate test
# What does this PR do?

Adds `is_flaky` decorator to the `test_run_squad_no_trainer` test which occasionally fails on CI runs which are independent  to the changes in the PR e.g.: 

* https://app.circleci.com/pipelines/github/huggingface/transformers/49621/workflows/14c25312-58a5-4b0b-8b41-6c5bec668043/jobs/593213
* https://app.circleci.com/pipelines/gh/huggingface/transformers/49224/workflows/fbae76ab-9259-4695-bb06-475357172587/jobs/589262


## 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?

_The documentation is not available anymore as the PR was closed or merged._<|||||>Great! Feel free to merge and open an issue to track it :)<|||||>I've opened an issue here: https://github.com/huggingface/transformers/issues/19733