* It is used to create an operator library. It handles to split CPU and
  GPU sources and links operator common libraries.
* It also give a reasonable warning and error when operator developer
  not correctly implement an operator.
  * Warning for lack of GPU kernel.
* Same interface as `cc_library` to make code style consistent.

* Convert `op` --> `operators`
* Remove AddType in OpProtoMaker, because type is part of registry.
* Rename CPU_OR_GPU --> DEVICE_TYPE in registry macro.

* Refine register methods, make Op can get rid of whole-archieve
* `USE_OP` before a op is used.
* Add unittest for add_op.