Avoid complex-to-real cast warning in CopyBackward (#60021)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/60021
Dropping the imaginary component is expected and gives the correct gradient
formula, so silencing the warning is appropriate.
Test Plan: Imported from OSS
Reviewed By: ngimel
Differential Revision: D29589371
Pulled By: mruberry
fbshipit-source-id: 73e1511cae69207dc9abe576e2769ee1d03f1bbd
