Remove backward ops for miopen depthwise convolution (#70064)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/70064
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D33171169
Pulled By: jbschlosser
fbshipit-source-id: 668ca9baa992d3bb1cfa7b53fd2127ffeb051147
