message: Add raise statement before NotImplementedError
diff: @@ -68,7 +68,7 @@ class Aggregator(object):
  Arguments:
  batch_outs: A list of batch-level outputs.
  """
  - NotImplementedError('Must be implemented in subclasses.')
  + raise NotImplementedError('Must be implemented in subclasses.')
  @abc.abstractmethod
  def aggregate(self, batch_outs, batch_start=None, batch_end=None):
  @@ -81,12 +81,12 @@ class Aggregator(object):
  batch_end: The end index of this batch. Always `None` if `use_steps` is
  `True`.
  """
  - NotImplementedError('Must be implemented in subclasses.')
  + raise NotImplementedError('Must be implemented in subclasses.')
  @abc.abstractmethod
  def finalize(self):
  """Prepares the total results to be returned."""
  - NotImplementedError('Must be implemented in subclasses.')
  + raise NotImplementedError('Must be implemented in subclasses.')
  class MetricsAggregator(Aggregator):
  