pin_memory should not copy on already pinned tensors (#23484)
Summary:
fixes https://github.com/pytorch/pytorch/issues/21076
Pull Request resolved: https://github.com/pytorch/pytorch/pull/23484
Differential Revision: D16546264
Pulled By: ezyang
fbshipit-source-id: 8058e0bbc6336751f36b884d71234feef498a982
