code: def dummy_callback(self):
      return CustomCallback()
  
imports: import os
  from collections import Counter
  import pytest
  import torch
  from pythae.models import AE, RAE_L2, VAEGAN, Adversarial_AE, Adversarial_AE_Config, AEConfig, RAE_L2_Config, VAEGANConfig
  from pythae.trainers import *
  from pythae.trainers.training_callbacks import *
label_desc: Use image node_id 3 for calling the Test_TrainersCalls obj's underlying member method code with example usage: obj.dummy_callback() and returns: CustomCallback