Add Wav2Vec2 Adapter Weights to Flax
# What does this PR do?

Fixes #15476

- Adds an adapter to the Flax Wav2Vec2 model to reduce the time dimension of the extracted feature vectors beyond that of the standard Wav2Vec2 model. The encoder's output hidden states thus have a time context window that is more similar to that of a subword token instead of just a character. 
- Shape and values of Flax output logits match those of the PyTorch model.
- Flax model uses all PyTorch model weights, including those of the adapter. Running the script in #15476 resolved to yield identical results (within 4e-2 threshold).
_The documentation is not available anymore as the PR was closed or merged._<|||||>I think the commit history is messed up here - the best is usually to just reopen a new PR and to just extract your changes from this PR.