code: def test_on_update_end(self):
      self.assertEqual(len(self.trainer.callbacks), 1)
      user_callback = self.trainer.callbacks[0]
      self.assertTrue(isinstance(user_callback, LRSchedulerCallback))
  
file_path: mmf/tests/trainers/callbacks/test_user_callback.py
imports: import argparse
  import os
  import unittest
  from copy import deepcopy
  import torch
  from mmf.common.registry import registry
  from mmf.trainers.callbacks.lr_scheduler import LRSchedulerCallback
  from mmf.utils.configuration import load_yaml
  from omegaconf import OmegaConf
  from tests.test_utils import NumbersDataset, SimpleModel
label_desc: Use image node_id 3 for calling the TestUserCallback obj's underlying member method code with example usage: obj.test_on_update_end() without return types