docstr: Utility function to get the Tensorboard URL
docstr_tok: ["Utility","function","to","get","the","Tensorboard","URL"]
code: def tensorboard_url(self):
      """Utility function to get the Tensorboard URL"""
      for node in self.cluster_info:
          if node["tb_port"] != 0:
              return "http://{0}:{1}".format(
                  node["host"], node["tb_port"]
              )
      return None
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import nested_scopes
  from __future__ import print_function
  import logging
  import os
  import random
  import signal
  import sys
  import threading
  import time
  from pyspark.streaming import DStream
  from .None import reservation
  from .None import TFManager
  from .None import TFSparkNode
label_desc: Use image node_id 4 for calling the TFCluster obj's underlying member method code with example usage: obj.tensorboard_url() and returns: None, str