some principle in the new paddle

• An operator should not create Variable in it's run Scope. Op/kernel should only use scope->GetVariable and judge if the return value is nullptr.
• In most cases, Variables should be initialized by python.
• Sometimes(such as RNN op) Operator can create child scopes, and it can create Variables in the scope created by itself.
• If Op wants to store some states in Scope as Variable, it should add them to inputs/outputs of this Op, and Python will help to create the Variables.