Parse_if multi broadcast output for empty shape branches instead of adding outline

This makes sure we're getting the correct output value of the branch if an
empty shape is given as a parameter to the IF operand.

src/onnx/parse_if.cpp

@@ -99,13 +99,19 @@ struct parse_if : op_parser<parse_if>
             // need to update the then_shape before we do further checks
             if(then_shape.size() == 0)
             {
-                auto convert_ins = then_mdl->add_outline(else_out_shapes.at(0));
+                auto convert_ins = then_mdl->insert_instruction(
+                    --else_mdl->end(),
+                    make_op("multibroadcast", {{"out_lens", else_shape}}),
+                    {--(--then_mdl->end())});
                 then_mdl->replace_return({convert_ins});
                 then_shape = else_shape;
             }
             else if(else_shape.size() == 0)
             {
-                auto convert_ins = else_mdl->add_outline(then_out_shapes.at(0));
+                auto convert_ins = else_mdl->insert_instruction(
+                    --else_mdl->end(),
+                    make_op("multibroadcast", {{"out_lens", then_shape}}),
+                    {--(--else_mdl->end())});
                 else_mdl->replace_return({convert_ins});
                 else_shape = then_shape;
             }