Merge pull request #27902 from abhishek-gola:onehot_layer_add

Added Onehot layer support to new DNN engine #27902

### 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

@@ -556,6 +556,13 @@ CV__DNN_INLINE_NS_BEGIN
         static Ptr<GridSampleLayer> create(const LayerParams& params);
     };
 
+    class CV_EXPORTS OneHotLayer : public Layer
+    {
+    public:
+        int axis;
+        static Ptr<OneHotLayer> create(const LayerParams& params);
+    };
+
     class CV_EXPORTS FlattenLayer : public Layer
     {
     public: