Serializer supports new pad api (#2739)

* Serializer supports new pad api

* Addressed reviewer comment

* Fixed compiler warning

src/ngraph/serializer.cpp

@@ -981,11 +981,15 @@ static shared_ptr<ngraph::Function>
                 // This is a legacy field whose functionality is no longer supported. The new
                 // behavior is equivalent to interior padding of 0, so we will accept it under
                 // those conditions.
-                auto padding_interior = node_js.at("padding_interior").get<vector<size_t>>();
+                auto padding_interior_maybe = node_js.find("padding_interior");
+                if (padding_interior_maybe != node_js.end())
+                {
+                    auto padding_interior = padding_interior_maybe->get<vector<size_t>>();
                     NGRAPH_ASSERT(std::all_of(padding_interior.begin(),
                                               padding_interior.end(),
                                               [](size_t s) { return s == 0; }))
                         << "Legacy padding_interior field must be zero everywhere.";
+                }
 
                 auto pad_mode = node_js.count("pad_mode") == 0
                                     ? op::PadMode::CONSTANT