Remove backward ops for NNPACK spatial convolution (#70305)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/70305
Test Plan: Imported from OSS
Reviewed By: zou3519
Differential Revision: D33279223
Pulled By: jbschlosser
fbshipit-source-id: f263012b3edaa87ce5430ffd6204a5453360d5dd
