code: def on_epoch_end(self, epoch, logs=None):
      if hvd.rank() == 0:
          log_state(self.state)
  
          current_hosts = epoch_to_hosts.get(
              self.state.epoch, default_hosts
          )
          next_hosts = epoch_to_hosts.get(
              self.state.epoch + 1, default_hosts
          )
          if args.discovery_wait > 0 and current_hosts != next_hosts:
              print(
                  "host changes: {} -> {}".format(
                      current_hosts, next_hosts
                  )
              )
              start = int(time.time())
              while state._host_messages.empty():
                  if int(time.time()) - start > args.discovery_wait:
                      raise TimeoutError(
                          "Timed out waiting for notifications from driver."
                      )
                  time.sleep(0.1)
  
      if args.epoch_wait > 0:
          time.sleep(args.epoch_wait)
  
file_path: horovod/test/integration/data/elastic_tensorflow_keras_main.py
imports: import argparse
  import json
  import os
  import psutil
  import time
  import tensorflow
  import horovod.tensorflow.keras
label_desc: Use image node_id 4 for calling the TestCallback obj's underlying member method code with example usage: obj.on_epoch_end(epoch, logs) without return types