* Support AucRunner in PaddleBox
* update some code style

* Add InitializeGPUAndLoadModel to solve random hang when downloading sparse parameters.
* Update SaveBase to solve test problem.

* Involves AfsAPI to resolve slow downloading.
* Mainly used in PaddleBox

* Add two types of Metric Calculator: MultiTaskCalculator & CmatchRankCalculator.
* Add a config for DynamicAdjustChannelNum function to denote whether we will discard the remaining instances when they are not be distributed evenly.
* Remove CPU code in Pull/PushSparse and we will add it back when testing it fully.
* Fix some known issues: such as copying persistable vars after one epoch running.

* Add a single_process_multi_thread transpiler.
* Add some UTs.
* Fix some API description.

* Support looking up embeddings from BoxPS.
* Add a _pull_box_sparse op, for now this op is not exposed to users.
* Add a BoxHelper class, providing 'BeginPass', 'EndPass', 'FeedPass' functions and so on.
* Add 'BoxPSDataset' in python code.
* Add a compile options WITH_BOX_PS and a MACRO PADDLE_WITH_BOX_PS.
* Add UT.
* More concrete information pls refer to: https://github.com/PaddlePaddle/Paddle/pull/18982

test=develop

test=develop

test=develop