Merge pull request #28957 from abhishek-gola:fusion_block_layout_extension

Extend attention fusion for runtime QK scale and add MatMul to Gemm rewriter #28957

### Pull Request Readiness Checklist

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- [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

@@ -1800,6 +1800,7 @@ CV__DNN_INLINE_NS_BEGIN
         bool trans_b;
         float alpha;
         float beta;
+        bool flatten_a;
 
         static Ptr<GemmLayer> create(const LayerParams& params);
     };
@@ -1808,6 +1809,8 @@ CV__DNN_INLINE_NS_BEGIN
      public:
         bool trans_a;
         bool trans_b;
+        float alpha;
+        float beta;
 
         static Ptr<MatMulLayer> create(const LayerParams &params);
     };