class_docstr: A test NNOperator with no functionality.
code: def __call__(self):
      engine_log.info("I'm an NNOperator.")
  
imports: from towhee.operator import NNOperator
  from towhee.utils.log import engine_log
label_desc: Use image node_id 4 for calling the TestNNOperator obj's underlying member method code with example usage: obj.__call__() without return types