Make `binary_cross_entropy` differentiable wrt `target` (#59447)
Summary:
As per title. Resolves https://github.com/pytorch/pytorch/issues/56683.
`gradgradcheck` will fail once `target.requires_grad() == True` because of the limitations of the current double backward implementation.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/59447
Reviewed By: agolynski
Differential Revision: D28910140
Pulled By: albanD
fbshipit-source-id: 20934880eb4d22bec34446a6d1be0a38ef95edc7
