Force empty branches to be an outline.

Outline of an empty branch is correct typically seen if we parse in an empty
constant for the other side of the if then/else onnx logic.

Need to do this to match shape even though the other branch is just empty.

src/onnx/parse_if.cpp

@@ -95,24 +95,21 @@ struct parse_if : op_parser<parse_if>
                 throw_shapes();
             }
 
-            auto handle_empty_branch = [](module_ref& mdl, std::vector<size_t>& out_shape) {
-                auto convert_ins =
-                    mdl->insert_instruction(--mdl->end(),
-                                            make_op("multibroadcast", {{"out_lens", out_shape}}),
-                                            {--(--mdl->end())});
-                mdl->replace_return({convert_ins});
+            auto handle_empty_branch = [](module_ref& mdl, const shape& out_shape) {
+                auto outline_ins = mdl->add_outline(out_shape);
+                mdl->replace_return({outline_ins});
             };
 
             // Handle one empty branch by setting output identical to the other
             // need to update the then_shape before we do further checks
             if(then_shape.empty())
             {
-                handle_empty_branch(then_mdl, else_shape);
+                handle_empty_branch(then_mdl, else_out_shapes.at(0));
                 then_shape = else_shape;
             }
             else if(else_shape.empty())
             {
-                handle_empty_branch(else_mdl, then_shape);
+                handle_empty_branch(else_mdl, then_out_shapes.at(0));
                 else_shape = then_shape;
             }
             else