Backprop Test for Freeze FlaxWav2Vec2 Feature Encoder
This PR correctly implements a back propagation test to verify the functionality of the `freeze_feature_encoder` argument added to the FlaxWav2Vec2 Model in #15873. It tests:

1. That the computed loss for the frozen feature encoder model and unfrozen model are **equal**.
2. That the gradients of the frozen feature encoder **differ** to those of the unfrozen feature encoder.
3. That the gradients of all other unfrozen layers remain **equal**.
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_15938). All of your documentation changes will be reflected on that endpoint.<|||||>If @patil-suraj is happy with this test I'll merge!