Merge pull request #29104 from abhishek-gola:sdpa

Added SDPA layer (Scaled Dot Product Attention) #29104

Merge with: https://github.com/opencv/opencv_extra/pull/1374

### 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

@@ -1872,6 +1872,12 @@ CV__DNN_INLINE_NS_BEGIN
         static Ptr<AttentionLayer> create(const LayerParams &params);
     };
 
+    // Scaled-dot-product attention on pre-projected, pre-reshaped Q / K^T / V.
+    class CV_EXPORTS SDPALayer : public Layer {
+     public:
+        static Ptr<SDPALayer> create(const LayerParams &params);
+    };
+
     class CV_EXPORTS RotaryEmbeddingLayer : public Layer {
      public:
         static Ptr<RotaryEmbeddingLayer> create(const LayerParams &params);