HifiGAN MelSpectrogram Vocoder Model
This pull request **introduces 2 alerts** when merging 666b2f0424ed018b360d614556d69303eace460c into 4c1d7b586fe764894f3468a11369aebc4962fa9f - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-45a9c8c1c8841634c3bfb490060da2fd2d84f798)

**new alerts:**

* 2 for Unused import please check DCO's bot output and sign unsigned commits > please check DCO's bot output and sign unsigned commits

done
This PR introduces the HifiGAN vocoder to NeMo.