Make ImageProcessorMixin compatible with subfolder kwarg
Adds subfolder support to the ImageProcessorMixin to be able to load models from a specific subfolder on the HuggingFace repo.

See the issue: https://github.com/huggingface/transformers/issues/21715
_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger I have added the test. Let me know your thoughts please.