Make a function that logs the duration since it was made.

def make_relative_timing_fn():
  """Make a function that logs the duration since it was made."""
  start_time = time.time()

  def format_relative_time():
    time_delta = time.time() - start_time
    return str(datetime.timedelta(seconds=time_delta))

  def log_relative_time():
    tf.logging.info("Timing: %s", format_relative_time())

  return log_relative_time