message: Remove Dummy layer
diff: #ifdef HAVE_DNN_NGRAPH
  #include "../ie_ngraph.hpp"
  #include <ngraph/op/experimental/layers/detection_output.hpp>
  -
  -namespace ngraph {
  -namespace op {
  -
  -class Dummy : public Op {
  -public:
  - Dummy() : Op("Dummy", {}) {
  - constructor_validate_and_infer_types();
  - }
  -
  - void validate_and_infer_types() override {
  - set_output_type(0, ngraph::element::Type(), {});
  - }
  -
  - std::shared_ptr<Node> copy_with_new_args(const NodeVector& new_args) const override {
  - if (!new_args.empty())
  - throw ngraph_error("Incorrect number of new arguments");
  - return std::make_shared<Dummy>();
  - }
  -
  - static constexpr NodeTypeInfo type_info{"Dummy", 1};
  - const NodeTypeInfo& get_type_info() const override {
  - return type_info;
  - }
  -};
  -
  -constexpr NodeTypeInfo Dummy::type_info;
  -
  -} // namespace op
  -} // namespace ngraph
  #endif
  namespace cv
  @@ -1000,10 +970,8 @@ public:
  attrs.code_type = std::string{"caffe.PriorBoxParameter." + _codeType};
  attrs.normalized = true;
  - auto aux_class_preds = std::make_shared<ngraph::op::Dummy>();
  - auto aux_box_preds = std::make_shared<ngraph::op::Dummy>();
  auto det_out = std::make_shared<ngraph::op::DetectionOutput>(box_logits, class_preds,
  - proposals, aux_class_preds, aux_box_preds, attrs);
  + proposals, attrs);
  return Ptr<BackendNode>(new InfEngineNgraphNode(det_out));
  }
  #endif // HAVE_DNN_NGRAPH
  