message: Stop running np.median if no callbacks are run.
diff: @@ -194,6 +194,8 @@ class CallbackList(object):
  def _call_batch_hook(self, mode, hook, batch, logs=None):
  """Helper function for all batch_{begin | end} methods."""
  + if not self.callbacks:
  + return
  hook_name = 'on_{mode}_batch_{hook}'.format(mode=mode, hook=hook)
  if hook == 'begin':
  self._t_enter_batch = time.time()
  