[TTS] Add modules for mel spectrogram codec
This PR is stale because it has been open for 14 days with no activity. Remove stale label or comment or update or this will be closed in 7 days. jenkins jenkins jenkins > What about model class file? Would it be added later?

This is trained using the same `audio_codec.py` model class. jenkins
# What does this PR do ?

Add code and example for how to train audio codec while encoding mel spectrogram.

Note there is an extra "FullBandMelEncoder" definition which is not in the example config, but which I am adding to try to and keep inference working with some existing checkpoints.

**Collection**: [TTS]

# Changelog 
- Add codec modules for encoding mel spectrogram.
- Add example config file.

# Jenkins CI
To run Jenkins, a NeMo User with write access must comment `jenkins` on the PR.

# 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?
- [ ] 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
- [ ] Documentation
