Update loss module doc (#48596)
Summary:
Fixes #{issue number}
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48596
Reviewed By: izdeby
Differential Revision: D25889748
Pulled By: zou3519
fbshipit-source-id: 9f6e77ba2af4030c8b9ae4afcea6d002a4dae423
