Remove backward op for 2d depthwise convolution (#70461)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/70461
Test Plan: Imported from OSS
Reviewed By: bdhirsh
Differential Revision: D33340494
Pulled By: jbschlosser
fbshipit-source-id: f2d8b2fcf9ad0f42b644b1dba51a694d83975566
