[PyTorch][easy] Fix borrowing from optional in binary_cross_entry_with_logits
Pull Request resolved: https://github.com/pytorch/pytorch/pull/78266
Saves a refcount bump.
Differential Revision: [D36650626](https://our.internmc.facebook.com/intern/diff/D36650626/)
Approved by: https://github.com/jbschlosser
