* add hdfs ls retry time and sleep time, fix save inference
* test=develop

* fix bug with half communicator

* add GeneralRoleMaker which is for general usage
* test=develop

* add half_async in the communicator
* fix DistributedStrategy

* add distributed_strategy

* fix fleet save bug of save_infernece_model
* test=develop

* fix some typos in AMP. test=develop

* delete useless codes. test=develop

* print table stat
test=develop

* notes
test=develop

* notes
test=develop

* fix fs_client_param bug， user can set this config through fleet_desc_file or fleet config
* test=develop

* general table

* add sparse table
test=develop

* no cvm
test=develop

* add no_cvm
test=develop

* add note
test=develop

* code style
test=develop

* code style
test=develop

* code style
test=develop

* code style
test=develop

* code style
test=develop

* add key of optimizer
test=develop

* solve pslib stop core
test=develop

* barrier
test=develop

* add notes
test=develop

* fix fleet util bug in save paddle inference model
* test=develop

* general table

* add sparse table
test=develop

* no cvm
test=develop

* add no_cvm
test=develop

* add note
test=develop

* code style
test=develop

* code style
test=develop

* code style
test=develop

* code style
test=develop

* code style
test=develop

* add key of optimizer
test=develop

test=develop

* fix cache table bug
* add save_paddle_inference_model
* fix hdfs util bug
* test=develop

* copy some feasigns and corresponding embeddings from one sparse table to another
* copy all feasigns and corresponding embeddings from one sparse table to another
* copy all dense params from one table to another
* copy some local vars to other local vars

modify the implementation of save_persistables and save_inference_model for fleet collective mode (#20802)

* modify the implementation of  save_persistables and save_inference_model functions for fleet collective, test=develop

* add ut, test=develop

test=develop

* fix PaddleCloud Role maker & add warning in distribute transpiler  & change rpc_retry_times

* support dump param to afs
test=develop

* code style
test=develop

* code style
test=develop

* dump param
test=develop

* dump param
test=develop

* dump param
test=develop

* dump param
test=develop

* no longer need to define all embedding layers (no one less) of all slots in each program. make trainer_param repeated in ps.proto.
* add find_distributed_lookup_table_grads instead of hard code GRAD
* support embedding stop gradient. push sparse has error before fix this.* 
* fix fill sparse, skip slots which do not have embedding. each slot's embedding in a sparse table should be used in all training programs before fix this.
* fix pull sparse, skip slots which do not have embedding.
* fix collect feasign label info, skip slots which do not have embedding.
* support when there are multi sparse tables in one or multi training programs, each program can pull/push its own related sparse tables instead of all sparse tables.
* test=develop

* add check nan / inf in downpour worker during training
* test=develop

* test=develop,Fix communicator slow bug

* test=develop, delete if() in stop_worker()

* test=develop

* fix UT, test=develop

* fix bug in fetch handler, test=develop

* fix bug in fetch handler, test=develop

* test=develop, fix fetch barrier bug

* test=develop, bug fix

* test=develop, bug fix

* test=develop, fix bug

* special case: strategy is None

* support dump multi file
test=develop

* dump fix num file
test=develop

* fix fc sort . test=develop

* refector geo sgd & communicator

* support change shuffle thread num
* support change train thread num
* fix receive shuffle data of each channel
* data norm stop gradient
* add check thread_tensor type and root_tensor type when merge metric
* remove sleep in shuffle, add config
* add config of pslib client to client communication
* fix xbox str
* add data norm op testcase
* add flush in trainer finalize

* add recompute based checkpoints methods for large batch training
test=develop

* add append_backward_with_forward_recomputation
test=develop

* refine optimizer
test=develop

* update backward and optimizer
test=develop

* make Variable usable
test=develop

* add recompute code

* refine optimizer
test=develop

* refine addup _append_backward_ops_with_checkpoints_
1) for recompute part, just cache the grad_op_desc without appending to block
2) before appending grad_op_desc to backward part, addup_repetitive_vars, remove unused branch
test=develop

* make method private

* add recompute strategy into DistributedStrategy
test=develop

* checkpoint version3
test=develop

* remove some print information
test=develop

* remove unused sumop
test=develop

* try to fix recompute with graph building modules

* add input names to vars should be held

* add memory debug tool

* backup backward

* Fix bugs

* add backward desc for op not in any segments

* add exception info for sub_block

test=develop

* modify code style

test=develop

* modify code style

test=develop

* remove print functions

test=develop

* add API spec

test=develop
test=document_preview

* make Recompute a child class of Optimizer

test=develop
test=document_preview

* add API spec

test=develop
test=document_preview

* modify API spec

test=develop
test=document_preview

* add document for Recompute

test=develop
test=document_preview

* change API doc of Rcompute

test=develop
test=document_preview

* code cleaning

test=develop
test=document_preview

* modify API spec

* fix bugs when segments hold no element

* add testcase for Recompute Optimizer

test=develop
test=document_preview

* add test for apply_gradient, and code cleaning

test=develop
test=document_preview

* add test case for load function

* enable CI

test=develop
test=document

* add test case

test=develop
test=document_preview

* add sample code for 4 function of recompute optimizer

test=develop
test=document_preview

* optimize cloud rolemaker, test=develop

change _origin_program test=develop

* support preload thread
* sleep before fleet wrapper exit for pslib core dump
* optimize hdfs log
* fix master+patch bug

Fix float16 optimizer

* fleet api add input check, test=develop

* test=develop,  communicator merge add => merge average