[SR] Eliminate no-ops (#67437)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67437
Certain ops do nothing on the forward pass and can be discarded after training: `aten::detach` and `fb::scale_gradient` are examples of this.
Test Plan: `buck test caffe2/test:jit -- test_freezing`
Reviewed By: hlu1
Differential Revision: D31980843
fbshipit-source-id: 0045b6babcfae786a2ce801b2f5997a078205bc0
