Fix SmoothL1Loss when target.requires_grad is True. (#44486)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44486
SmoothL1Loss had a completely different (and incorrect, see #43228) path when target.requires_grad was True.
This PR does the following:
1) adds derivative support for target via the normal derivatives.yaml route
2) kill the different (and incorrect) path for when target.requires_grad was True
3) modify the SmoothL1Loss CriterionTests to verify that the target derivative is checked.
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D23630699
Pulled By: gchanan
fbshipit-source-id: 0f94d1a928002122d6b6875182867618e713a917
