Port `nll_loss_backward` to structured (#62144)
Summary: Pull Request resolved: https://github.com/pytorch/pytorch/pull/62144
Test Plan: Imported from OSS
Reviewed By: heitorschueroff
Differential Revision: D29945279
Pulled By: SplitInfinity
fbshipit-source-id: 2fee60e8424fc590a81767c9b0a8226a0c2fd69c
