message: Make tf_should_use warnings less verbose.
  Limit the stack trace to 5 frames.
diff: @@ -65,7 +65,8 @@ class _TFShouldUseHelper(object):
  else:
  logger = tf_logging.error
  creation_stack = ''.join(
  - [line.rstrip() for line in traceback.format_stack(self._stack_frame)])
  + [line.rstrip()
  + for line in traceback.format_stack(self._stack_frame, limit=5)])
  logger(
  '==================================\n'
  'Object was never used (type %s):\n%s\nIf you want to mark it as '
  