[Wav2Vec2] Rename model's feature extractor to feature encoder (#14959)
* rename classes
* clean up more namings
* remove bogus file
* Apply suggestions from code review
* Apply suggestions from code review
* replace more names
* more regex replace
* make style
* correct
* correct more
* make style
* finish
* correct more in wav2vec2
* make style
* improve freeze_extractor
* add aliases
* add tf aliases