message: Add AggregatingVariable.value method
diff: @@ -155,6 +155,9 @@ class AggregatingVariable(variables_lib.Variable, core.Tensor):
  def op(self):
  return self._v.op
  + def value(self):
  + return self._v.value()
  +
  def read_value(self):
  return self._v.read_value()
  