[ONNX] Fix dtype for log_softmax export (#46627)
Summary:
Previously dtype was not converted from constant node to python number properly.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/46627
Reviewed By: houseroad
Differential Revision: D24657535
Pulled By: bzinodev
fbshipit-source-id: 33b0b9087d969f2cb0a2fa608fcf6e10956c06bf
