Another dce fix (pytorch#22499)

Summary:
Pull Request resolved: pytorch#22499

Another place where onnx export is running dead code elimination after making the jit graph invalid. Fixing it.

Reviewed By: houseroad

Differential Revision: D16111969

fbshipit-source-id: 5ba80340c06d091988858077f142ea4e3da0638c

torch/onnx/utils.py

@@ -351,7 +351,7 @@ def _model_to_graph(model, args, verbose=False, training=False,
 
     if do_constant_folding and _export_onnx_opset_version == 9:
         params_dict = torch._C._jit_pass_onnx_constant_fold(graph, params_dict)
-        torch._C._jit_pass_dce(graph)
+        torch._C._jit_pass_dce_allow_deleting_nodes_with_side_effects(graph)
 
     if verbose:
         print(graph)