code: def state_key(self):
      return type(self)
  
file_path: pytorch-lightning/tests/tests_pytorch/callbacks/test_callbacks.py
imports: from pathlib import Path
  from re import escape
  from unittest.mock import Mock
  import pytest
  from lightning.pytorch import Callback, Trainer
  from lightning.pytorch.callbacks import ModelCheckpoint
  from lightning.pytorch.demos.boring_classes import BoringModel
  from lightning_utilities.test.warning import no_warning_call
label_desc: Use image node_id 2 for calling the OldStatefulCallback obj's underlying member method code with example usage: obj.state_key() and returns: type