Make FlaxPreTrainedModel a Flax Module
Moved to #22627 
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_22479). All of your documentation changes will be reflected on that endpoint.<|||||>Thanks for the review @sanchit-gandhi !

> => perhaps as a start we first make a PR that triggers this warning (advising users that the functinality is going to change in N months / releases time), and then have this PR as a follow-up that makes the changes?

I was planning on entirely deleting the `params` argument from `__call__` 😅. If we want to make the change a bit more gradual then maybe we could do something like this:

```diff
- if self._do_init:
+ if self.scope is None:
```
This would condition on whether the module being called inside `apply` or not.<|||||>Closing in favor of #22627<|||||>Thanks for the clarifications here @cgarciae! 🙌