* replace part of the old implementation, test=develop
* restore concat op, test=develop
* update all ops implemention & delete GetDataTypeOfVar func, test=develop
* Rewrite is_empty op to avoid unnecessary data transform.
test=develop
* Add the implementation of InferShape and InferVarType for is_empty op.
test=develop
* Rewrite is_empty op to avoid directly inherit OperatorBase.
test=develop
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.