Remove backward op for slow 3d convolution (#69978)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/69978
Test Plan: Imported from OSS
Reviewed By: zou3519
Differential Revision: D33131003
Pulled By: jbschlosser
fbshipit-source-id: 097440b2eb501c1eeeb8a666d4bc3508fc5d0cfa
