[fix] cross_entropy : fix weight with ignore_index and label_smoothing (#69511)
Summary:
Fixes https://github.com/pytorch/pytorch/issues/69339
cc albanD mruberry jbschlosser walterddr
Pull Request resolved: https://github.com/pytorch/pytorch/pull/69511
Reviewed By: mrshenli
Differential Revision: D32951935
Pulled By: jbschlosser
fbshipit-source-id: 482eae851861a32f96bd6231dd3448fb6d44a015
