Remove backward op for slow 2d transposed convolution (#70333)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/70333
Test Plan: Imported from OSS
Reviewed By: bdhirsh
Differential Revision: D33301402
Pulled By: jbschlosser
fbshipit-source-id: 3cfb3165589fe1620f22479b05139676d20dc493
