message: NotImplementedError needs to be raised
diff: @@ -219,7 +219,7 @@ class Metric(Layer):
  *args:
  **kwargs: A mini-batch of inputs to the Metric.
  """
  - NotImplementedError('Must be implemented in subclasses.')
  + raise NotImplementedError('Must be implemented in subclasses.')
  @abc.abstractmethod
  def result(self):
  @@ -228,7 +228,7 @@ class Metric(Layer):
  Result computation is an idempotent operation that simply calculates the
  metric value using the state variables.
  """
  - NotImplementedError('Must be implemented in subclasses.')
  + raise NotImplementedError('Must be implemented in subclasses.')
  ### For use by subclasses ###
  @doc_controls.for_subclass_implementers
  