Replace deprecated pytorch methods

[lipeng31]

• torch.cuda.amp.GradScaler(...) => torch.amp.GradScaler("cuda", ...)
• torch.cuda.amp.autocast(...) => torch.amp.autocast("cuda", ...)

[csukuangfj]

could you make it backward compatible?

[lipeng31]

I read the PyTorch docs and found the first version that provides both torch.cuda.amp.autocast and torch.cuda.amp.GradScaler is 1.6.0, and the first version that provides torch.amp.autocast("cuda", ...) and torch.amp.GradSclaer("cuda", ...) is 1.10.0 which was released 3 years ago, so the compatibility problem happens only for the versions of [1.6.0,1.10.0). I'm not sure if this icefall is supposed to support those versions of PyTorch. Additionally, maintaining compatibility with those versions of PyTorch would require numerous changes throughout the codebase, potentially making the code less clean. Could you reconsider the necessity of this compatibility? Thanks.

[lipeng31]

Last commit fails some checks. I'll fix it soon.