Fix resizing bug in OWL-ViT
# What does this PR do?
Fixes a resizing issue in `OwlViTFeatureExtractor` that lead to the image/s getting resized along only one dimension and getting cropped along the other dimension later on in the preprocessing pipeline. The issue was due to defining the size as a single value instead of a tuple (768 instead of (768, 768)). 

The configuration files are updated and the `OwlViTProcessor `can correctly resize the input images now. This PR changes the default target `size` and sets default `do_center_crop` argument as False.

Fixes # 18553

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- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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- [x] Did you write any new necessary tests?

@NielsRogge could you take a look?




_The documentation is not available anymore as the PR was closed or merged._<|||||>> Thanks for fixing! However, all "owlvit" checkpoints on the hub have `"do_center_crop": true` in their preprocessor_config.json.
> 
> Should we update them after this PR is merged?

Good point, yes, we should update them after the merge