GitLab

* first version of refactoring reduce operators.

* clang format

* refactor the gpu implemantation of the reduce_mean operator

* clang format

* refactor gpu implementation of the resuce_sum operator

* fix cpp check error

* fix cppcheck error

* fix cppcheck error

* fix review comments

* clang format

* fix a jenkin error

* fixed review comments

* clang format

* fix review comments

* clang format

* fix review comments

* clang format

* add implemenation of reduce_min and reduce_max

* clang format

* add unit test for reduce_min/max operator

* clang format

* add more unit tests

* clang format

* fix review comments