Fixes default dtype value for onnx hardtanh export (opset11) (#35467)
Summary:
Oneline fix to lara-hdr 's PR https://github.com/pytorch/pytorch/pull/30169.
Default `dtype` value should be set when `dtype is None` rather than when `dtype is not None`.
I didn't make an issue for this as such a small change but I have been using this locally in order to export a model with opset 11 (opset 10 still works).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/35467
Differential Revision: D20686048
Pulled By: mruberry
fbshipit-source-id: 726a5f9c0711c7a79b171fe98b602cdef27f9b31
