class_docstr: Subtract operator (used for unit test purposes only).
      
code: def __call__(
      self, a: int, b: int
  ) -> NamedTuple("Outputs", [("diff", int)]):
      Outputs = NamedTuple("Outputs", [("diff", int)])
      return Outputs(a - b)
  
file_path: towhee/tests/unittests/mock_operators/sub_operator/sub_operator.py
imports: from typing import NamedTuple
  from towhee.operator import Operator, SharedType
label_desc: Use image node_id 2 for calling the SubOperator obj's underlying member method code with example usage: obj.__call__(a, b) and returns: Outputs