Fix `AutoImageProcessor` requiring torchvision when only Pillow is installed (#48616)
* Do not infer a torchvision requirement from prose mentions of TorchvisionBackend
* Test the ImportError rather than the file text
Replaces the two import-structure tests with one that rebuilds the auto
module with torchvision patched unavailable and asserts AutoImageProcessor
still resolves.
* test each model class
* clean-up the patch
* skip it, old API
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Co-authored-by: raushan <raushan@huggingface.co>