Add the SEW and SEW-D speech models
# What does this PR do?

This PR adds the SEW and SEW-D model from the paper "[Performance-Efficiency Trade-offs in Unsupervised Pre-training for Speech Recognition](https://arxiv.org/abs/2109.06870)" 

Source of the models: https://github.com/asappresearch/sew/

* **SEW** is based on Wav2Vec2, but with time frame downsampling and upsampling around the transformer layers.
* **SEW-D** replaces the transformer layers in SEW with a DeBERTa-v2 encoder.

## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in this PR.

## TODO
- [x] model docs
- [x] checkpoints conversion
- [ ] fintetuned CTC checkpoints?
The VQ pretraining modules aren't ported yet. After #13877 is merged they'll be added in a separate PR.<|||||>Let's try to get this PR merged by Thursday/Friday - anything I can help with? :-)<|||||>@patrickvonplaten in the end I removed the `feature_projection` if-else and left the modules only in SEW-D.
The checkpoints are all uploaded now :tada: 
https://huggingface.co/models?other=sew
https://huggingface.co/models?other=sew-d