Add EnCodec model
Author: hollance
Pull request: yes
Labels: name: New model

# What does this PR do?

Adds the EnCodec neural codec from the [High Fidelity Neural Audio Compression](https://arxiv.org/abs/2210.13438) paper.

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Comments:
What still needs to be done at this point:

- remove `assert`s
- remove `einops` stuff: `rearrange`, `repeat`
- `QuantizedResult` -> `QuantizerOutput`
- I don't like the names in the `EncodecOutput` etc classes, so left some TODO items for renaming them
- rename arguments in config file (see TODO items in that file)
- add doc comments for all arguments in config file
- clean up `_linear_overlap_add`, the padding functions, and `_kmeans`
- improve variable names in the modeling code
- get rid of `activation_params` from config
- fix issues with padding on the 48khz model, since the last (incomplete) frame is different than with the original model
- remove all training code; I suggest we throw an exception from all `forward` methods
- doc strings in modeling code
- MDX file
- and probably more stuff

Most of these are small items.

TODO list:

**Done**
- 
 remove asserts 

- 
 remove einops stuff: rearrange, repeat 

- 
 QuantizedResult -> QuantizerOutput 

- 
 I don't like the names in the EncodecOutput etc classes, so left some TODO items for renaming them 

- 
 clean up _linear_overlap_add, the padding functions, and _kmeans 

- 
 remove all training code; I suggest we throw an exception from all forward methods 

- 
 improve variable names in the modeling code 

- 
 get rid of activation_params from config 

- 
 rename arguments in config file (see TODO items in that file) 

- 
 add doc comments for all arguments in config file 


**Final TODO**:
- doc strings in modeling code
- MDX file
- fix issues with padding on the 48khz model, since the last (incomplete) frame is different than with the original model

_The documentation is not available anymore as the PR was closed or merged._

Thanks for the review. Before asking for a final review:
- [x] Finish designing the modelling integration tests.
- [x] Finish testing all edge cases for the feature extractor.
Working on this! 

Ok, addressed everything! feel free to merge if it's good with you @amyeroberts