Fix test_pipelines_video_classification that was always failing
Author: CalOmnie
Pull request: yes

# What does this PR do?

`tests/test_pipelines_video_classification.py` was always failing because the output of the video classifier, when run with one file path (as opposed to a list), is returning a `list[dict]` whereas the testing code was expecting a `list[list[dict]]`, this PR corrects that assumption.

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
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- [ ] 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.
It would be nice if someone could check that `pytest tests/test_pipelines_video_classification.py` actually fails for them too, it is possible that the issue comes from my environment

Comments:
cc @ydshieh, can you confirm this test was failing?

@CalOmnie I am not able to see  `test_pipelines_video_classification.py` failing on our CI report.

Could you provide a link to a job run page where this test is failing or a full command that run a specific test that shows this failure?

Hey @ydshieh thanks for taking the time to go over this PR. I shared this command in the PR description but I probably should have made a dedicated section for it, I ran the tests using:
```
pytest tests/test_pipelines_video_classification.py
```
I assume it is not part of your automated testing, or it would have been found earlier. That or an issue with my own environment.

OK, it should be `tests/pipelines/test_pipelines_video_classification.py`. But with that, I can't see it from the Slack report as the message is truncated. 

This test is indeed failing on `main` and fixed in this PR.

I find the docstring for `VideoClassificationPipeline.__call__` in `src/transformers/pipelines/video_classification.py` is misleading:

```python
        Return:
            A dictionary or a list of dictionaries containing result. If the input is a single video, will return a
            dictionary, if the input is a list of several videos, will return a list of dictionaries corresponding to
            the videos.
```

Even with a single video (path string), the returned value is still a list of dictionary instead of the mentioned `dictionary` in the above docstring. (It's a list of length `top_k`).

@CalOmnie  Would you like to double check this part and update the docstring if necessary?

@Rocketknight1 The changes in this PR LGTM.



@ydshieh Apologies for the wrong path. I updated the docstring to reflect that the pipeline now returns `top_k` results, even with a single video.

@Rocketknight1  Maybe you can double check the change in `src/transformers/pipelines/video_classification.py` and then we are ready to go?