make `gather` cpu kernel to be multiple threads. (#12246)

* Change the gather op to multi-thread.

* Modify the gather kernel of xla compiler in order to be compatible with multi-threads cpu kernel.

* Add prefetch logic to gather op kernel.

* Update the indention of gather op kernel code.

* Update the gather kernel code for multiple thread.

* Remove reference to ealier version of code in gather functor.

* Change the framework_lite dep of gather_functor to framework.

* Remove mutex guard in gather functor.