[TTS] Add license to ported EnCodec code
# What does this PR do ?

Move code ported from EnCodec github into encodec_modules and add MIT Meta license to it.

https://github.com/facebookresearch/encodec/blob/main/LICENSE

**Collection**: [TTS]

# Changelog 
- Copy/paste code from audio_codec_modules.py and vector_quantization.py into encodec_modules.py
- Add MIT license

# 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)
- [ ] 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**:
- [ ] New Feature
- [ ] Bugfix
- [x] Documentation