Merge pull request #3996 from vpisarev:matshape_inside_mat

Fixed compile errors after MatShape/MatSize refactoring #3996

this is satellite for https://github.com/opencv/opencv/pull/27757

### 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
- [ ] 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.
- [ ] The feature is well documented and sample code can be built with the project CMake

modules/gapi/samples/pipeline_modeling_tool/utils.hpp

@@ -38,7 +38,7 @@ inline void generateRandom(cv::Mat& out) {
             break;
         case CV_16F: {
             std::vector<int> dims;
-            for (int i = 0; i < out.size.dims(); ++i) {
+            for (int i = 0; i < out.size.dims; ++i) {
                 dims.push_back(out.size[i]);
             }
             cv::Mat fp32_mat;