GitLab

Summary:
Part 1 of [T138011314](https://www.internalfb.com/intern/tasks/?t=138011314)

This PR ports the generator part of [HiFi GAN](https://arxiv.org/abs/2010.05646v2) from [the original implementation](https://github.com/jik876/hifi-gan/blob/4769534d45265d52a904b850da5a622601885777/models.py#L75)

Adds tests:
- Smoke tests for architectures V1, V2, V3
- Check that output shapes are correct
- Check that the model is torchscriptable and scripting doesn't change the output
- Check that our code's output matches the original implementation. Here I clone the original repo inside `/tmp` and import necessary objects from inside the test function.  On test teardown I restore `PATH`, but don't remove the cloned code, so that it can be reused on subsequent runs - let me know if removing it would be a better practice

There are no quantization tests, because the model consists mainly of `Conv1d` and `ConvTransposed1d`, and they are [not supported by dynamic quantization](https://pytorch.org/docs/stable/quantization.html)

Pull Request resolved: https://github.com/pytorch/audio/pull/2860

Reviewed By: nateanl

Differential Revision: D41433416

Pulled By: sgrigory

fbshipit-source-id: f135c560df20f5138f01e3efdd182621edabb4f5