* cache core.ops, test=develop

* refine, test=develop

* add scatter-gather opt

* fix topo for pp

* rename function

* kill process by group

* opt dygraph python code, test=develop

* refine, test=develop

* pipeline adaptive for heterps;test=develop
* fix finalize hang;test=develop
* add is_compiled_with_heterps for dataset;test=develop
* fix hashtable core when pass ins_num=0;test=develop

* add amp support for pp

* add amp untest

* fix hdfs and fleet_util for supporting save/load infer;test=develop

* add random and prevent deadlock

* dygraph sharding

* update unitest hybrid_parallel_communicate_group

* elastic unitest

* rename demo

* del py2 code2

* fix test timeout

* elastic etcd ready

* add pplayer

* add sharedlayerdesc

This reverts commit b2afc8df.

* update spawn doc for xpu, test=ducument_fix

* add note for gpu and xpu, test=document_fix

* update 2.0 public api in all left files

* reverse device.py all list;
fix some flake8 errors

* fix Undefined variables

* fix Undefined variables

* update, test=develop

* add parallel_cross_entropy

* add grad for crossentropy

* fix cross entropy