[Wav2Vec2] Better names for internal classes
Wav2Vec2's classes have too many names, *e.g.*: FlaxWav2Vec2EncoderLayerStableLayerNormCollection. 

We should make those names easier (reminder for myself @patrickvonplaten to do this)
This also concerns a badly chosen duplicate of `Wav2Vec2FeatureExtractor` as part of `modeling_wav2vec2.py` since it's also the "FeatureExtractor" in `feature_extraction_wav2Vec2.py` -> should be corrected as well.<|||||>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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