Add current_device() to torch.cpu (#110987)
Better support device agnostic, add a "cpu" return for `current_device()` in torch.cpu so that we won't run into `AttributeError: module 'torch.cpu' has no attribute 'current_device'`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/110987
Approved by: https://github.com/wanchaol
