Support automatically Name Loop Variable in IRBuilder (#716) (#741)

* [SCHEDULE]enable partition const loop with build flag (#719)

    * enable partition loop with build flag

    * add a testcase, and modify LoopPartition related cases

*     * add document for split_const_loop

* [IRbuild]Support automatically Name Loop Variable in IRBuilder (#719)

    * add idx_num in class

* using typical index [i, j, k] first, then i_suffix

* keep inputs names

* fix lint

* improve comment of name

* fix lint