class_docstr: This class handles iteration events to log summaries to
  the TensorBoard log.
docstr: Initialise summary writers.
  
  :param _sender:
  :param msg:
  :return:
code: def init_writer(self, _sender, **_unused_msg):
      """
      Initialise summary writers.
  
      :param _sender:
      :param msg:
      :return:
      """
      # initialise summary writer
      if not self.summary_dir or self.tensorboard_every_n <= 0:
          return
      self.writer_train = tf.summary.FileWriter(
          os.path.join(self.summary_dir, TRAIN), tf.get_default_graph()
      )
      self.writer_valid = tf.summary.FileWriter(
          os.path.join(self.summary_dir, VALID), tf.get_default_graph()
      )
  
file_path: NiftyNet/niftynet/engine/handler_tensorboard.py
imports: import os
  import tensorflow
  from niftynet.engine.application_variables import TF_SUMMARIES
  from niftynet.engine.signal import TRAIN, VALID, ITER_STARTED, ITER_FINISHED, GRAPH_CREATED
  from niftynet.io.misc_io import get_latest_subfolder
label_desc: Use image node_id 2 for calling the TensorBoardLogger obj's underlying member method code with example usage: obj.init_writer(_sender) without return types