Remove backward op for slow 3d transposed convolution (#69933)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/69933
Test Plan: Imported from OSS
Reviewed By: bdhirsh
Differential Revision: D33131343
Pulled By: jbschlosser
fbshipit-source-id: 4300c66f0f4811c949f82c62d17c7b5200cd15a3
