Do not use ctor

* Reduce line of codes.
* We can use virtual function for Maker now.
* The implementation does not care what maker holds, it is easier to
refactor later.

* Temporarily fix bug for backward tanspiler when using parallel_do operator.

* Fix bug for backward tanspiler when using parallel_do operator

* Refine target_assign_op to unify the classification and regression targets assignment.

* Fix the unit testing.

* Fix conflicts.