Convert ONNX ops to new form (#3343)

src/ngraph/frontend/onnx_import/core/null_node.cpp

@@ -14,17 +14,16 @@
 // limitations under the License.
 //*****************************************************************************
 
-#include "null_node.hpp"
+#include <string>
+
 #include "ngraph/node.hpp"
+#include "null_node.hpp"
 
 namespace ngraph
 {
     namespace onnx_import
     {
-        NullNode::NullNode()
-            : ngraph::Node("NullNode", {}, 0)
-        {
-        }
+        const std::string NullNode::type_name{"NullNode"};
 
         std::shared_ptr<Node> NullNode::copy_with_new_args(const NodeVector& new_args) const
         {

src/ngraph/frontend/onnx_import/core/null_node.hpp

@@ -36,7 +36,10 @@ namespace ngraph
         class NullNode : public ngraph::Node
         {
         public:
-            NullNode();
+            NGRAPH_API
+            static const std::string type_name;
+            const std::string& description() const override { return type_name; }
+            NullNode() = default;
 
             bool is_null() const final override { return true; }
             virtual std::shared_ptr<Node>