ONNX Export Support for CrossEntropyLoss (#34830)
Summary:
Add ONNX export support for torch.nn.CrossEntropyLoss.
This PR makes following changes:
1. Updates nll_loss export
2. Makes a post pass for SoftmaxCrossEntropy
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34830
Reviewed By: hl475
Differential Revision: D21230712
Pulled By: houseroad
fbshipit-source-id: c81911a41968e23813ba10274340ce4d8ba1ed78
