Nnaudio

Refactored most of the code so that it supports `stft_layer.to(device)` like other torch modules now, where `stft_layer = nnAudio.Spectrogram.STFT()` or any other spectrogram class available in nnAudio.

Since this version no longer uses the device argument, it is not compatible with the older versions. Adjustments are required to upgrade your code from older versions to `0.2.0`.

For example, `nnAudio.Spectrogram.STFT(device=device)` in the older version should be rewritten as `nnAudio.Spectogram.STFT().to(device)` in `0.2.0`.

If you are using nnAudio inside your PyTorch model, then `model.to(device)` is enough to transfer nnAudio layers to the device you want.

This is the first stable release for documentation using the `sphinx-versioning` extension for `Sphinx`.
Tag is necessary for `sphinx-versioning` to create another page for previous nnAudio versions.