Merge pull request #28963 from abhishek-gola:custom_layers

Added custom layer support in new DNN engine #28963

Closes: https://github.com/opencv/opencv/issues/26200
Merge with: https://github.com/opencv/opencv_extra/pull/1358

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [x] There is a reference to the original bug report and related work
- [x] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [x] The feature is well documented and sample code can be built with the project CMake

modules/dnn/src/onnx/onnx_importer2.cpp

@@ -894,11 +894,12 @@ void ONNXImporter2::parseNode(const opencv_onnx::NodeProto& node_proto)
                 << node_proto.output_size() << " outputs from domain '"
                 << layer_type_domain << "'");*/
 
+    // Unknown domain: still try parseCustomLayer; addLayer() handles the miss.
     if (dispatch.empty())
     {
-        CV_LOG_ERROR(NULL, "DNN/ONNX: missing dispatch map for domain='" << layer_type_domain << "'");
-        rememberMissingOp(layer_type);
-        return;
+        CV_LOG_DEBUG(NULL, "DNN/ONNX: no built-in dispatch map for domain='"
+                     << layer_type_domain << "', trying custom layer for '"
+                     << layer_type << "'");
     }
 
     node_inputs.clear();
@@ -990,8 +991,21 @@ void ONNXImporter2::parseNeg(LayerParams& layerParams, const opencv_onnx::NodePr
     addLayer(layerParams, node_proto);
 }
 
+// Lookup unknown ONNX ops in the user-side LayerFactory; non-default
+// domains are prefixed (e.g. "my.namespace.MyOp"), matching the old engine.
 void ONNXImporter2::parseCustomLayer(LayerParams& layerParams, const opencv_onnx::NodeProto& node_proto)
 {
+    const std::string& name = layerParams.name;
+    std::string& layer_type = layerParams.type;
+    const std::string& layer_type_domain = node_proto.has_domain() ? node_proto.domain() : std::string();
+
+    if (!LayerFactory::isLayerRegistered(layer_type))
+    {
+        CV_LOG_INFO(NULL, "DNN/ONNX: unknown node type '" << layer_type
+                << "' (domain '" << layer_type_domain << "', node '" << name
+                << "'). Register a handler via CV_DNN_REGISTER_LAYER_CLASS() or LayerFactory::registerLayer().");
+    }
+
     parseSimpleLayers(layerParams, node_proto);
 }