Fix the derivative of `acosh` for complex numbers. (#80841)
For a summary of what was going on, see https://github.com/pytorch/pytorch/issues/50692#issuecomment-1173922441
Fixes https://github.com/pytorch/pytorch/issues/50692
Pull Request resolved: https://github.com/pytorch/pytorch/pull/80841
Approved by: https://github.com/albanD
