handle onnx nll with default ignore index (#44816)
Summary:
in ONNX NegativeLogLikelihoodLoss specification, ignore_index is optional without default value.
therefore, when convert nll op to ONNX, we need to set ignore_index attribute even if it is not specified (e.g. ignore_index=-100).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/44816
Reviewed By: ezyang
Differential Revision: D23880354
Pulled By: bzinodev
fbshipit-source-id: d0bdd58d0a4507ed9ce37133e68533fe6d1bdf2b
