Freeze FlaxWav2Vec2 Feature Encoder
This PR enables the FlaxWav2Vec2 Feature Encoder to be _frozen_. The flow of gradients through the Feature Encoder during forward or reverse-mode automatic differentiation is prevented through use of a simple command (`jax.lax.stop_gradient`), thus holding the Feature Encoder parameters fixed. Freezing the Feature Encoder is required for the majority of FlaxWav2Vec2 fine-tuning set-ups, making this PR an important addition to the FlaxWav2Vec2Model.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15873). All of your documentation changes will be reflected on that endpoint.<|||||>As discussed offline - the test didn't check what it was supposed to check. Bad review from my part. We should open a new PR here to fix it cc @patil-suraj (sorry should have put you as a reviewer as well)