Fixes default dtype value for onnx hardtanh export (opset11) (#35467)

Summary:
Oneline fix to lara-hdr 's PR #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: #35467

Differential Revision: D20686048

Pulled By: mruberry

fbshipit-source-id: 726a5f9c0711c7a79b171fe98b602cdef27f9b31

torch/onnx/symbolic_opset11.py

@@ -21,7 +21,7 @@
 @parse_args('v', 'f', 'f')
 def hardtanh(g, self, min_val, max_val):
     dtype = self.type().scalarType()
-    if dtype is not None:
+    if dtype is None:
         dtype = 6  # float
     else:
         dtype = sym_help.scalar_type_to_onnx.index(sym_help.cast_pytorch_to_onnx[dtype])