class_docstr: Visitor which counts the graph nodes.
code: def visit(self, operation_def: OperationDef, _) -> Tuple[int]:
      self.num_nodes += 1
      return tuple(1 for _ in range(operation_def.num_outputs))
  
imports: import abc
  import collections
  import dataclasses
  from typing import Any, Collection, Dict, List, Optional, Tuple
  import pydot
label_desc: Use image node_id 1 for calling the _CountGraphNodes obj's underlying member method code with example usage: obj.visit(operation_def, _) and returns: tuple