Image processor mixin

[amyeroberts]

What does this PR do?

This is part of a series of PRs to introduce the ImageProcessor class for vision models.

Adds an alias for the FeatureProcessorMixin as ImageProcessorMixin. This mixin provides the saving/loading functionality for the processing classes. Much of the logic will be exactly the same between the two, and so a more general class is needed for feature extractors and image processors. Otherwise we are likely to have confusing inheritance in the code e.g. class XxxImageProcessor(FeatureExtractorMixin.

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline,
  Pull Request section?
• Was this discussed/approved via a Github issue or the forum? Please add a link
  to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

[HuggingFaceDocBuilderDev]

The documentation is not available anymore as the PR was closed or merged.

[alaradirik]

Looks good to me!