ADD hifi-gan

[kyakuno]

https://github.com/jik876/hifi-gan
Fast MelSpectrum2Wave that also runs on CPU.

[kyakuno]

Currently, tacotron2 uses waveglow, but the processing is heavy. Therefore, I would like to be able to use hifi-gan as an option.

[kyakuno]

An explanatory article about hifi gan.
https://qiita.com/NoriakiOshita/items/02deaa9af5dbbca02add

[kyakuno]

1. Register as hifi gan on ailia-models as a single model.
2. Allow it to be specified as an option to tacotron2.
   https://github.com/axinc-ai/ailia-models/tree/master/audio_processing/tacotron2

[YToleubay]

1. added HIFIGAN model (standalone) #1288 it is for task 1, Register hifi gan on ailia-models as a single model.
2. added HIFI GAN model (tacotron2) #1287 it is for task 2, Allow it to be specified as an option to tacotron2.

[kyakuno]

We have exported LJ_FT_T2_V1. I checked the paper, it was the model that provided the highest scores.