Low code coverage on latest develop branch

[typhoonzero]

How to generate the coverage:

1. Modify coveralls.cmake CMAKE_BUILD_TYPE to Release, this can be fixed soon.
2. Run cmake -DWITH_COVERAGE=ON -DCOVERALLS_UPLOAD=ON [other options] .., must compile WITH_GPU=ON since CPU only unit test fails on some operators. #7546 is not fixed.
3. Run make coveralls. It will probably fail because it's not running on a CI system.
4. Run pip install cpp-coveralls to install a tool to upload coveralls data.
5. Run coveralls --exclude CMakeFiles --exclude go --exclude proto --exclude python --exclude third_party --exclude Testing --gcov-options '\-lp' --dump new_coveralls.json to get a newly updated json
6. Modify the .json file to remove service_job_id and service_name field, and add repo_token of PaddlePaddle (Ask PaddlePaddle admin for it.).
7. Run curl -X POST -F json_file=@new_coveralls.json https://coveralls.io/api/v1/jobs
8. Visit https://coveralls.io/github/PaddlePaddle/Paddle to see detailed information.

The problem

Current coverage is generated using gconv, and fluid uses python to test c++ code, we can not get coverage across languages. So need to find a way to get it more accurate.