Torchsynth

This update includes some bug fixes and an update to the `Signal` class to enable checkpointing.

What's Changed
* Adding the drum nebula to docs by jorshi in https://github.com/torchsynth/torchsynth/pull/374
* Sphinx fixs by turian in https://github.com/torchsynth/torchsynth/pull/379
* Codecov Action Fix by jorshi in https://github.com/torchsynth/torchsynth/pull/380
* Updating torch.range to torch.arange by jorshi in https://github.com/torchsynth/torchsynth/pull/378 - `torch.range`, which is deprecated, was producing incorrect values for larger batch_ids passed into a synth voice.
* Fix floordiv by turian in https://github.com/torchsynth/torchsynth/pull/382
* Add new_empty to Signal by turian in https://github.com/torchsynth/torchsynth/pull/384 - this enables `deepcopy` on torchsynth Signals and allows for checkpointing


**Full Changelog**: https://github.com/torchsynth/torchsynth/compare/v1.0.1...v1.0.2

- Now works with torch 1.9

* All AbstractSynth now return multi-modal tuples: (audio batch, parameter batch, is_train batch)
* Batch sizes that are multiples of 32 are now supported for any reproducible output. (128 is still the default.)
* More detailed documentation, including documentation fix proposed by daisukelab.
* Default voice nebula added, as well as a drum nebula.
* modulation signal input on VCO and LFO is now optional

* Fixes bug that prevented importing of torchsynth without the README.md file
* Pushed preliminary docs

* Memory optimizations
* Preliminary docs