message: add trainable property to AggregatingVariable
diff: @@ -1421,6 +1421,10 @@ class AggregatingVariable(variables_lib.Variable):
  def name(self):
  return self._v.name
  + @property
  + def trainable(self):
  + return self._v.trainable
  +
  @property
  def dtype(self):
  return self._v.dtype
  