Created by: zhiqiu

As the title, this PR Polishes the usage of deprecated or private API in dygraph, i.e., APIs start with _.

• _ivar is removed since PR21359, we split VarBase (which is used in dygraph mode) from Variable (which is used in static mode). to_variable and layers.xx will return an VarBase object, Variable does not hold _ivar any more.
• _stop_gradient is a private API, use stop_gradient instead.