Improve ONNX symbolic for logsoftmax and softmax (#17672)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17672
support dtype in the onnx symbolic
Reviewed By: zrphercule
Differential Revision: D14313987
fbshipit-source-id: e9364621b3f795191d880599711dfbcb220d0e31
