Fix valid ratio for Deformable Detr
# What does this PR do?

I encountered unexpected behavior that single-batch image and multi-batch image return significantly different output.
I found that its reason is from function `get_valid_ratio`, which returns ratio of image size for each example from batch (which is padded for longest width and height in batch). Since mask has opposite value(True for real pixel, False for padded) from original repo, mask should be opposite with [original repo](https://github.com/fundamentalvision/Deformable-DETR/blob/main/models/deformable_transformer.py#L117-L124) inside of function `get_valid_ratio`. Otherwise it will return image ratio for **pad width and height**, which is obviously erroneous.


## 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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- [ ] Did you write any new necessary tests?


## Who can review?

@NielsRogge 


_The documentation is not available anymore as the PR was closed or merged._<|||||>cc @NielsRogge and @amyeroberts 