Return the string message associated with TensorBoard purges.

def _GetPurgeMessage(most_recent_step, most_recent_wall_time, event_step,
                     event_wall_time, num_expired):
  """Return the string message associated with TensorBoard purges."""
  return ('Detected out of order event.step likely caused by a TensorFlow '
          'restart. Purging {} expired tensor events from Tensorboard display '
          'between the previous step: {} (timestamp: {}) and current step: {} '
          '(timestamp: {}).'
         ).format(num_expired, most_recent_step, most_recent_wall_time,
                  event_step, event_wall_time)