Add complex support for torch.mean [CUDA] (#47048)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/46982
Pull Request resolved: https://github.com/pytorch/pytorch/pull/47048
Reviewed By: heitorschueroff
Differential Revision: D24729895
Pulled By: anjali411
fbshipit-source-id: 8e948480eb87c37de810207edf909375c0380772
