Remove bool argument, use a class to handle that.

OLD: op_type -> grad_op_creator

NEW: grad_op_type -> grad_op_creator
     op_type -> grad_op_type

* Refine unittest also

Replace `OperatorPtr` with `std::shared_ptr<OperatorBase>`

* ScopePtr means pointer of scope, but it can be shared or uniqued.
Change it to std::shared_ptr<Scope> to make code better to read.