Fix MSELoss when target.requires_grad is True. (#44437)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44437
MSELoss had a completely different (and incorrect, see https://github.com/pytorch/pytorch/issues/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 MSELoss CriterionTests to verify that the target derivative is checked.
TODO:
1) do we still need check_criterion_jacobian when we run grad/gradgrad checks?
2) ensure the Module tests check when target.requires_grad
3) do we actually test when reduction='none' and reduction='mean'?
Test Plan: Imported from OSS
Reviewed By: albanD
Differential Revision: D23612166
Pulled By: gchanan
fbshipit-source-id: 4f74d38d8a81063c74e002e07fbb7837b2172a10
