Support gather export with OneHot + Mul (#21235)
Summary:
This could serve as a alternative solution to export ```torch.gather``` before something similar goes into ONNX spec. The exported model is verified to be correct against onnxruntime backend. We weren't able to test against Caffe2 backend because it doesn't seem to support OneHot opset9.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21235
Differential Revision: D15613039
Pulled By: houseroad
fbshipit-source-id: 7fc097f85235c071474730233ede7d83074c347f
