[ONNX] Fix in overriding ops (#2477)

* [ONNX] Overriding custom ops

* Add UT

* Style Check

* Review & style fix

src/ngraph/frontend/onnx_import/ops_bridge.cpp

@@ -140,9 +140,14 @@ namespace ngraph
                                                  const std::string& domain,
                                                  Operator fn)
         {
-            auto result = m_map[domain][name].emplace(version, std::move(fn));
-            if (result.second)
+            auto it = m_map[domain][name].find(version);
+            if (it == std::end(m_map[domain][name]))
             {
+                m_map[domain][name].emplace(version, std::move(fn));
+            }
+            else
+            {
+                it->second = std::move(fn);
                 NGRAPH_WARN << "Overwriting existing operator: "
                             << domain + "." + name + ":" + std::to_string(version);
             }

test/models/onnx/override_op.onnx

+ONNXnGraphImporter:c
+
+
A
+
B
C"FalseAdd
compute_graphZ
+
A
+

+
+Z
+
B
+

+
+b
+
C
+

+
+B

\ No newline at end of file

test/onnx_import.in.cpp

@@ -2031,6 +2031,33 @@ TEST(onnx_${BACKEND_NAME}, model_where)
     EXPECT_EQ(expected_outputs.front(), outputs.front());
 }
 
+TEST(onnx_${BACKEND_NAME}, model_override_op)
+{
+    onnx_import::register_operator(
+        "FalseAdd", 1, "", [](const onnx_import::Node& node) -> NodeVector {
+            NodeVector ng_inputs{node.get_ng_inputs()};
+            return {std::make_shared<ngraph::op::Add>(ng_inputs.at(0), ng_inputs.at(1))};
+        });
+
+    onnx_import::register_operator(
+        "FalseAdd", 1, "", [](const onnx_import::Node& node) -> NodeVector {
+            NodeVector ng_inputs{node.get_ng_inputs()};
+            return {std::make_shared<ngraph::op::Subtract>(ng_inputs.at(0), ng_inputs.at(1))};
+        });
+
+    auto function = onnx_import::import_onnx_model(
+        file_util::path_join(SERIALIZED_ZOO, "onnx/override_op.onnx"));
+
+    Inputs inputs;
+    inputs.emplace_back(std::vector<float>{0.f, 1.f, 2.f, 3.f});
+    inputs.emplace_back(std::vector<float>{3.f, 2.f, 1.f, 0.f});
+
+    Outputs expected_output{std::vector<float>{-3.f, -1.f, 1.f, 3.f}};
+
+    Outputs outputs{execute(function, inputs, "${BACKEND_NAME}")};
+    EXPECT_TRUE(test::all_close_f(expected_output.front(), outputs.front()));
+}
+
 TEST(onnx_${BACKEND_NAME}, import_non_existing_file)
 {
     try