Fix nll_backward for negative weights (#64572)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64572
Fixes https://github.com/pytorch/pytorch/issues/64256
It also fixes an inconsistent treatment of the case `reduction = "mean"`
when the whole target is equal to `ignore_index`. It now returns `NaN`
in this case, consistently with what it returns when computing the mean
over an empty tensor.
We add tests for all these cases.
Test Plan: Imported from OSS
Reviewed By: ezyang
Differential Revision: D31116297
Pulled By: albanD
fbshipit-source-id: cc44e79205f5eeabf1efd7d32fe61e26ba701b52
