HifiGAN finetuning on synthetic mels
This pull request **introduces 2 alerts** when merging eeeebf02faa79e72f4921a91517cba26173669a1 into 16b87d1f7d22126b8eb471ca0538f75ade9620cf - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-aa22cf3bd77bdd09b203bef1f11fdcc603a8ffeb)

**new alerts:**

* 1 for Unused local variable
* 1 for Unused import
This PR introduces fine-tuning of HifiGAN on synthetic mels generated by a spectrogram generator model (for example, Tacotron2).