Created by: reyoung

In current code-base, the type of gradient operator does not need to be registered, since

• User should never create a gradient operator in Python by hand. A user should invoke Backward method with a forward operator to create gradient operators. So the type of gradient operator is not needed.
• If the gradient operator is composed of forwarding operators, we should use NetOp(or BlockOp in the future). The sample code is here.