Added HifiGAN to TTS_Inference notebook
This pull request **introduces 1 alert** when merging 021e3b10e8a6a6b27cf059ed6a6425bb2c5ffda8 into 05eb2587a95e512b5e95705d9f77582634abbd95 - [view on LGTM.com](https://lgtm.com/projects/g/NVIDIA/NeMo/rev/pr-30f680fe1081bb12260d2adaace8bd00410cb403)

**new alerts:**

* 1 for Unused import
This PR adds HifiGAN to the TTS inference notebook, and adds an NGC checkpoint trained on ground-truth LJSpeech mels to list of available models.