Flax Refactor v2
# What does this PR do?

Alternative to #22627. Instead of making `FlaxPretrainedModel` a Flax `Module`, this PR aims to make all inner `.module`s usable in a standalone way by moving any pre/posprocessing done by its `*Model` container into the Module itself.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_22866). All of your documentation changes will be reflected on that endpoint.<|||||>Hey @cgarciae - we're nearly finished with this PR far as I can tell. Do you have the bandwidth to see this through to completion?  Happy to help with the last stages of integration here!<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

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