Why we use `Debug` build type

[gongweibao]

# make sure CPU only build with openblas passes.
nvidia-docker run -i --rm -v $PWD:/paddle -v /root/.cache:/root/.cache\
    -e "APT_MIRROR=${apt_mirror}"\
    -e "WITH_GPU=OFF"\
    -e "WITH_AVX=%WITH_AVX%"\
    -e "WITH_GOLANG=%WITH_GOLANG%"\
    -e "WITH_TESTING=OFF"\
    -e "WITH_COVERAGE=OFF" \
    -e "COVERALLS_UPLOAD=OFF" \
    -e "GIT_PR_ID=${PR_ID}" \
    -e "WITH_C_API=%WITH_C_API%" \
    -e "CMAKE_BUILD_TYPE=Debug" \
    -e "WITH_MKL=OFF" \
    -e "JSON_REPO_TOKEN=JSUOs6TF6fD2i30OJ5o2S55V8XWv6euen" \
    -e "WITH_DEB=OFF"\
    -e "PADDLE_VERSION=%PADDLE_VERSION%"\
    -e "PADDLE_FRACTION_GPU_MEMORY_TO_USE=0.15" \
    -e "RUN_TEST=OFF" ${PADDLE_DEV_NAME}

[helinwang]

Thanks! I have changed it to RelWithDebInfo for builds in "PR CI". CC: @putcn