message: Fix writing binary summaries in python3.
  In python 3, tf.string tensors are bytes() objects when fetched, not str(). six.binary_type
  maps exactly to tf.string in python 2 and 3, so use it to determine if we should try to interpret
  a metric as a tf.Summary proto.
diff: @@ -1103,7 +1103,7 @@ def _write_dict_to_summary(output_dir,
  isinstance(dictionary[key], np.int32) or
  isinstance(dictionary[key], int)):
  summary_proto.value.add(tag=key, simple_value=int(dictionary[key]))
  - elif isinstance(dictionary[key], six.string_types):
  + elif isinstance(dictionary[key], six.binary_type):
  try:
  summ = summary_pb2.Summary.FromString(dictionary[key])
  for i, _ in enumerate(summ.value):
  