class_docstr: Subtract operator (used for unit test purposes only).
      
code: def input_schema(self):
      return [(int, (1,)), (int, (1,))]
  
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 4 for calling the SubOperator obj's underlying member method code with example usage: obj.input_schema() and returns: list