class_docstr: A hook to capture metadata for a single step.
  Useful for performance debugging. It performs a full trace and saves
  run_metadata and Chrome timeline information to a file.
  
  Args:
    step: The step number to trace. The hook is only enable for this step.
code: def begin(self):
      self._global_step = tf.train.get_global_step()
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  from __future__ import unicode_literals
  import abc
  import os
  import numpy
  import six
  import yaml
  import tensorflow
  from tensorflow.python.training.basic_session_run_hooks import SecondOrStepTimer
  from tensorflow.python.training import session_manager
  from tensorflow.python.client import timeline
  from tensorflow import gfile
  from seq2seq.configurable import Configurable, abstractstaticmethod
  from seq2seq import graph_utils, global_vars
label_desc: Use image node_id 3 for calling the MetadataCaptureHook obj's underlying member method code with example usage: obj.begin() without return types