Add support for train without specify pretrained model, add support for selecting v2 48k as training setting, and add support for auto remove pretrained model when the user do not have pretrained model in designate folder.

[JunityZhan]

Add support for train without specify pretrained model, add support for selecting v2 48k as training setting, and add support for auto removing pretrained model path in corresponding box when the users do not have pretrained model in designate folder. I have tested it on my machine and it works fine. If you have any further questions about this pull request, please let me know.