fix bug in mask2former: cost matrix is infeasible
Author: xuchenhao001
Pull request: yes

# What does this PR do?

As per the title, fix a bug that causes the error of ``ValueError: cost matrix is infeasible``.
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Fixes #21644


## 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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      [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
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@amyeroberts

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