Add GAN-based TTS Vocoder
This pull request **introduces 6 alerts** when merging 1a32266b9ae76f036aac21b58486d9e9c54ebe40 into 4de7bd8394fb7e809e04ee8bcadcf9a1711ef497 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-f4743e03b4be34541d11903c6d31e9d4f4d054b4)

**new alerts:**

* 3 for Unused import
* 1 for Signature mismatch in overriding method
* 1 for Unused local variable
* 1 for Suspicious unused loop iteration variable This pull request **introduces 3 alerts** when merging 31efecac501313a482045fa84f4c9de37b1c0a43 into 4de7bd8394fb7e809e04ee8bcadcf9a1711ef497 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-213f5a3610ecb1c635b1218b7044c6fe988b1630)

**new alerts:**

* 1 for Signature mismatch in overriding method
* 1 for Unused import
* 1 for Wrong number of arguments in a class instantiation This pull request **introduces 1 alert** when merging 1d83b438b70c1b622a1fdb773462fc4dfda432d8 into 4de7bd8394fb7e809e04ee8bcadcf9a1711ef497 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-da57d6a36fa0609eaa3daebd4bebdf361ce395c7)

**new alerts:**

* 1 for Signature mismatch in overriding method This pull request **introduces 1 alert** when merging ea45a80522c99426c3e70465af8bb5a087b52624 into 38c854da3f553b6577d6f8b08490e026b53b7ea3 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-8cbef5ba362dbc3ec63a86ec04acafd90e0fc464)

**new alerts:**

* 1 for Unused import
- Adds the fullband MelGAN model as described in https://arxiv.org/abs/2005.05106
- Remove empty *_step() from ModelPT
- Split UniGlowLoss into glow portion and STFTLoss portion
- Simply stft_exact_pad option in FilterBankFeatures


ToDos
- [x] ~Enable FP16~. No currently a priority.
- [x] Add NeMo Loss subclass
- [x] Update Docs
- [x] ~Update TTS tutorials~, can't until I upload the .nemo files