Added the exception of ignore_index (#18117)
Summary:
Fix #17801 to add an exception regarding `ignore_index` in the documentation for `torch.nn.CrossEntropyLoss` and `torch.nn.NLLLoss`
If any other files/functions are hit, I'd be glad to incorporate the changes there too! 😊
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18117
Differential Revision: D14542079
Pulled By: ezyang
fbshipit-source-id: 7b918ac61f441dde7d3d6782d080c500cf2097f1
