Google Summer of Code Support ONNX operator gather elements

[Aser-Abdelfatah]

Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

• I agree to contribute to the project under Apache 2 License.
• 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

This is a draft pull request for GSoC review purposes. Edits need to be made.

[asmorkalov]

@Aser-Abdelfatah Thanks for the PR. There are build issues on all platforms. Example:

FAILED: modules/dnn/CMakeFiles/opencv_dnn.dir/src/layers/gather_elements_layer.cpp.o 
/usr/bin/ccache /usr/bin/c++  -DCVAPI_EXPORTS -DCV_OCL4DNN=1 -DENABLE_PLUGINS -DHAVE_FLATBUFFERS=1 -DHAVE_PROTOBUF=1 -D_USE_MATH_DEFINES -D__OPENCV_BUILD=1 -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -I/home/ci/opencv/modules/dnn/include -Imodules/dnn -I/home/ci/opencv/modules/core/include -I/home/ci/opencv/modules/imgproc/include -I/home/ci/opencv/modules/dnn/misc/caffe -I/home/ci/opencv/modules/dnn/misc/tensorflow -I/home/ci/opencv/modules/dnn/misc/onnx -I/home/ci/opencv/modules/dnn/misc/tflite -I/home/ci/opencv/3rdparty/include/opencl/1.2 -isystem . -isystem /usr/include/eigen3 -isystem /home/ci/opencv/3rdparty/flatbuffers/include -isystem /home/ci/opencv/3rdparty/protobuf/src -fsigned-char -W -Wall -Wreturn-type -Wnon-virtual-dtor -Waddress -Wsequence-point -Wformat -Wformat-security -Wundef -Winit-self -Wpointer-arith -Wsign-promo -Wuninitialized -Wsuggest-override -Wno-delete-non-virtual-dtor -Wno-comment -Wimplicit-fallthrough=3 -Wno-strict-overflow -fdiagnostics-show-option -pthread -fomit-frame-pointer -ffunction-sections -fdata-sections    -fvisibility=hidden -fvisibility-inlines-hidden -Wno-deprecated -Wno-missing-declarations -Wno-shadow -Wno-unused-parameter -Wno-sign-compare -Wno-invalid-offsetof -O3 -DNDEBUG  -DNDEBUG -fPIC   -std=c++11 -MD -MT modules/dnn/CMakeFiles/opencv_dnn.dir/src/layers/gather_elements_layer.cpp.o -MF modules/dnn/CMakeFiles/opencv_dnn.dir/src/layers/gather_elements_layer.cpp.o.d -o modules/dnn/CMakeFiles/opencv_dnn.dir/src/layers/gather_elements_layer.cpp.o -c /home/ci/opencv/modules/dnn/src/layers/gather_elements_layer.cpp
/home/ci/opencv/modules/dnn/src/layers/gather_elements_layer.cpp: In member function 'virtual bool cv::dnn::GatherElementsImpl::getMemoryShapes(const std::vector<std::vector<int> >&, int, std::vector<std::vector<int> >&, std::vector<std::vector<int> >&) const':
/home/ci/opencv/modules/dnn/src/layers/gather_elements_layer.cpp:38:42: error: no matching function for call to 'std::vector<std::vector<int> >::assign(int, __gnu_cxx::__alloc_traits<std::allocator<int>, int>::value_type&)'
   38 |         outputs.assign(1, indicesShape[0]); // shape of output is same as indices

See details link in CI status box.