optimize the AMP func name in custom_device_mod (#98052)
Fixes #ISSUE_NUMBER
1、optimize the func name of AMP in custom device module，use `torch.foo.set_autocast_enable` instead of `torch.foo.set_autocast_foo_enable`.
2、In AMP with custom device，use `custom_device_mod.set_autocast_enable` instead of `getattr(custom_device_mod, "set_autocast_enable"`, because we have check that `custom_device_mod` hasattr `set_autocast_enable` before.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98052
Approved by: https://github.com/bdhirsh
