add bbox input validation
Author: jinhopark8345
Pull request: yes

# What does this PR do?

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`BrosModel` will only work if `bbox` is not `None` and currently there is no validation code for this case.
Add input validation code to the beginning of `BrosModel.forward` method as @ydshieh suggested. 


## 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?
- [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. https://github.com/huggingface/transformers/pull/23190#issuecomment-1727796291
- [ ] 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?


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@ydshieh 

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Comments:
All pass!

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