* The new vocoder Fre-GAN is now supported
* Improved some fregan details
* Fixed the problem that the existing model could not be loaded to continue training when training GAN
* Updated reference papers
* GAN training now supports DistributedDataParallel (DDP)
* Added requirements.txt
* GAN training uses single card training by default
* Added note about GAN vocoder training with multiple GPUs
* Added missing files for Fre-GAN
