fix nll loss decomposition to properly ignore ignore_index (#95833)
Fixes #95794
This is a hotfix for decomposition only (that is currently used by inductor), reference still accesses invalid indices. Perhaps `_nll_loss_nd` and this decomp should be unified, cc @soumith @voznesenskym @yanboliang @penguinwu @anijain2305 @EikanWang @jgong5 @Guobing-Chen @XiaobingSuper @zhuhaozhe @blzheng @Xia-Weiwen @wenzhe-nrv @jiayisunx @peterbell10 @desertfire @lezcano
Pull Request resolved: https://github.com/pytorch/pytorch/pull/95833
Approved by: https://github.com/lezcano, https://github.com/Chillee
