[ASR] Audio processing base, multi-channel enhancement models
# What does this PR do ?

This PR adds a base class for audio processing models, an enhancement model, a number of audio-related modules and tests.

**Collection**: ASR

# Changelog 
- Audio processing base model `AudioToAudioModel`
- `EncMaskDecAudioToAudioModel` subclass for speech enhancement
- Audio modules for spectrogram transforms, mask estimation based on RNN (`MaskEstimatorRNN`), mask-based processing (`MaskReferenceChannel` and `MaskBasedBeamforming`), `SDRLoss`
- Training script for `EncMaskDecAudioToAudioModel` in `speech_enhancement.py`
- Audio processing script for `AudioToAudioModel` in `process_audio.py`

# Usage
* You can potentially add a usage example below

```python
# Add a code snippet demonstrating how to use this 
```

# Before your PR is "Ready for review"
**Pre checks**:
- [x] Make sure you read and followed [Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md)
- [x] Did you write any new necessary tests?
- [x] Did you add or update any necessary documentation?
- [ ] Does the PR affect components that are optional to install? (Ex: Numba, Pynini, Apex etc)
  - [ ] Reviewer: Does the PR have correct import guards for all optional libraries?
  
**PR Type**:
- [x] New Feature
- [ ] Bugfix
- [x] Documentation

If you haven't finished some of the above items you can still open "Draft" PR.


## Who can review?

Anyone in the community is free to review the PR once the checks have passed. 
[Contributor guidelines](https://github.com/NVIDIA/NeMo/blob/main/CONTRIBUTING.md) contains specific people who can review PRs to various areas.

# Additional Information
* Related to #5196 
