Remove the redundant calculation of derivative of power function (#28651)
Summary:
Hi, I notice that the pytorch faced the the issue as HIPS/autograd#541 .
I try to solve it, hope it can help.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/28651
Reviewed By: gchanan
Differential Revision: D18137163
Pulled By: albanD
fbshipit-source-id: 888bef65c72c4c15c2acdd4b13d5041008b1354e
