docstr: Event called at the beginning of an epoch.
code: def on_epoch_begin(
      self, training_config: BaseTrainerConfig, **kwargs
  ):
      """
      Event called at the beginning of an epoch.
      """
      self.step_list.append("on_epoch_begin")
  
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 5 for calling the CustomCallback obj's underlying member method code with example usage: obj.on_epoch_begin(training_config) without return types