Add Vocos model
### Model description

Vocos is a Fourier-based neural vocoder for audio synthesis.

According to its [paper](https://arxiv.org/pdf/2306.00814.pdf), Vocos constantly outperforms [HifiGan](https://huggingface.co/docs/transformers/main/en/model_doc/speecht5#transformers.SpeechT5HifiGan), has 13.5M params and is significantly faster than any competing vocoders!

Moreover, it is also compatible with Bark, and significantly improve audio quality as showed [here](https://charactr-platform.github.io/vocos/#audio-reconstruction-from-bark-tokens).

Vocos is composed of a backbone (ConvNeXt) and an inverse fourier transform head (either STFT or MDCT).  


### Open source status

- [X] The model implementation is available
- [X] The model weights are available

### Provide useful links for the implementation

Vocos code is available [here](https://github.com/charactr-platform/vocos/tree/main) and was mainly contributed by @hubertsiuzdak.

Its weights are available on HF hub [here](https://huggingface.co/charactr/vocos-mel-24khz) and [here](https://huggingface.co/charactr/vocos-encodec-24khz).