Fix topk deserialize to be backwards compatible (#4171)

src/ngraph/serializer.cpp

@@ -2943,7 +2943,8 @@ shared_ptr<Node> JSONDeserializer::deserialize_node(json node_js)
         {
             auto compute_max = node_js.at("compute_max").get<bool>();
             auto target_type = read_element_type(node_js.at("index_element_type"));
-            op::TopKSortType sort = node_js.at("sort").get<op::TopKSortType>();
+            op::TopKSortType sort =
+                get_or_default<op::TopKSortType>(node_js, "sort", op::TopKSortType::SORT_VALUES);
             if (has_key(node_js, "top_k_axis"))
             {
                 auto top_k_axis = node_js.at("top_k_axis").get<size_t>();