add -Werror=return-type (#5655)

Signed-off-by: daquexian <daquexian566@gmail.com>

Co-authored-by: oneflow-ci-bot <69100618+oneflow-ci-bot@users.noreply.github.com>

CMakeLists.txt

@@ -127,7 +127,7 @@ if(WIN32)
   #set(CMAKE_EXE_LINKER_FLAGS_DEBUG "${CMAKE_EXE_LINKER_FLAGS} /DEBUG:FASTLINK")
   set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} /D_ITERATOR_DEBUG_LEVEL=0")
 else()
-  set(EXTRA_CXX_FLAGS "-std=c++11 -Wall -Wno-sign-compare -Wno-unused-function -fPIC")
+  set(EXTRA_CXX_FLAGS "-std=c++11 -Wall -Wno-sign-compare -Wno-unused-function -fPIC -Werror=return-type")
   if (APPLE)
      set(EXTRA_CXX_FLAGS "${EXTRA_CXX_FLAGS} -Wno-deprecated-declarations -Wno-mismatched-tags")
   endif()