Remove bool argument, use a class to handle that.

* Use EnforceNotMet to unify all exception types.

Make implement an operator less noisy.

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>`