Demucs

Release of Demucs v3, featuring hybrid domain separation and much more.
This drops support for Conv-Tasnet and training on the non HQ MusDB dataset.
There is no version 3.0.0 because I messed up.

- Fix in Tasnet (PR 178)
- Use ffmpeg in priority when available instead of torchaudio to avoid small shift in MP3 data.
- other minor fixes

MusDB HQ support added. Custom wav dataset support added.
Minor changes: issue with padding of mp3 and torchaudio reading, in order to limit that,
Demucs now uses ffmpeg in priority and fallback to torchaudio.
Replaced pre-trained demucs model with one trained on more recent codebase.

This is a big release, with at lof of breaking changes. You will likely
need to install Demucs from scratch.



- Demucs now supports on the fly resampling by a factor of 2.
This improves SDR almost 0.3 points.
- Random scaling of each source added (From Uhlich et al. 2017).
- Random pitch and tempo augmentation addded, from [Cohen-Hadria et al. 2019].
- With extra augmentation, the best performing Demucs model now has only 64 channels
instead of 100, so model size goes from 2.4GB to 1GB. Also SDR is up from 5.6 SDR to 6.3 when trained only on MusDB.
- Quantized model using [DiffQ](https://github.com/facebookresearch/diffq) has been added. Model size is 150MB, no loss in quality as far as I, or the metrics,
can say.
- Pretrained models are now using the TorchHub interface.
- Overlap mode for separation, to limit inconsitencies at
frame boundaries, with linear transition over the overlap. Overlap is currently
at 25%. Not that this is only done for separation, not training, because
I added that quite late to the code. For Conv-TasNet this can improve
SDR quite a bit (+0.3 points, to 6.0).
- PyPI hosting, for separation, not training!


Note: Versions in *italic* means that the release is a beta version.