update flake8 for config_file in projects (Oneflow-Inc#91)

Author: L1aoXingyu

.flake8

@@ -14,4 +14,5 @@ per-file-ignores =
   **/tests/config/**.py:F401,E402
   tests/config/**.py:F401,E402
   tests/**.py: E402
-  tools/**.py: E402
+  tools/**.py: E402
+  projects/**/configs/**.py:F401