message: call _validate() in _ParseOpParams.from_features().
diff: @@ -451,6 +451,7 @@ class _ParseOpParams(object):
  raise ValueError("Unsupported %s %s." %
  (type(feature).__name__, feature))
  params._add_feature(key, feature) # pylint: disable=protected-access
  + params._validate() # pylint: disable=protected-access
  return params
  @property
  