Remove backward ops for miopen convolution (#69987)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/69987
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D33136054
Pulled By: jbschlosser
fbshipit-source-id: e049168732bdfcf590ec8102412f2ef0418f9dcc
