Fix L1Loss when target.requires_grad is True. (#44471)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44471
L1Loss 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 L1Loss CriterionTests to verify that the target derivative is checked.
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D23626008
Pulled By: gchanan
fbshipit-source-id: 2828be16b56b8dabe114962223d71b0e9a85f0f5
