Merge pull request #27508 from abhishek-gola:if_layer_add

IfLayer add to new DNN engine #27508

### 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/include/opencv2/dnn/all_layers.hpp

@@ -585,6 +585,18 @@ CV__DNN_INLINE_NS_BEGIN
         static Ptr<RequantizeLayer> create(const LayerParams &params);
     };
 
+    // Forward declaration for computational Graph used by IfLayer
+    class Graph;
+
+    class CV_EXPORTS IfLayer : public Layer
+    {
+    public:
+        virtual int branch(InputArray arr) const = 0;
+
+        /** Factory: creates an IfLayer implementation. */
+        static Ptr<IfLayer> create(const LayerParams& params);
+    };
+
     class CV_EXPORTS ConcatLayer : public Layer
     {
     public: