Add complex autograd support for torch.mean (#47566)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/47566
Test Plan: Imported from OSS
Reviewed By: heitorschueroff
Differential Revision: D24817013
Pulled By: anjali411
fbshipit-source-id: f2b8411fb9abdc3e2d07c8e4fef3071b76605b12
