viewfinder-annn

Results 2 issues of viewfinder-annn

This PR is the completion for [issue#49](https://github.com/open-mmlab/Amphion/issues/49). Added detailed document for custom SVC dataset and instructions for training from existing sources(resume training/finetune). Fixed some bugs for custom dataset construction and...

**✨ Description:** This PR addresses bugs in VitsSVC module, mainly related to: 1. implement resume training / finetune feature 2. fix bug for multiple acoustic content training **🚧 Related Issues:**...