Merge pull request #25868 from Abdurrahheem:ash/add-gpt2-sample

Add sample for GPT2 inference #25868

### Pull Request Readiness Checklist

This PR adds sample for inferencing GPT-2 model. More specificly implementation of GPT-2 from [this repository](https://github.com/karpathy/build-nanogpt). Currently inference in OpenCV is only possible to do with fixed window size due to not supported dynamic shapes. 

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
- [ ] There is a reference to the original bug report and related work
- [ ] 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/dnn.hpp

@@ -728,7 +728,7 @@ CV__DNN_INLINE_NS_BEGIN
          *  @param outLayerShapes output parameter for output layers shapes;
          * order is the same as in layersIds
          */
-        void getLayerShapes(const MatShape& netInputShape,
+        CV_WRAP void getLayerShapes(const MatShape& netInputShape,
                                     const int& netInputType,
                                     const int layerId,
                                     CV_OUT std::vector<MatShape>& inLayerShapes,