Move Clip image utils to image_utils.py
- Use image_utils functions instead of duplicate functions in clip/feature_extraction_clip.py
- Move convert_rgb to image_utils.py

_The documentation is not available anymore as the PR was closed or merged._<|||||>Isn't this a breaking change? The CLIP method uses resize with `default_to_square=False`, right?

Yes, my mistake. Will fix it shortly!