* add sync communicator and implement

[cherry-pick] #22484

支持不依赖nccl进行编译。

多卡下，如果没有打开WITH_NCCL开关编译，则只能使用单卡

* Fix GEO-SGD init & send Bug (#22375)

* test=develop, fix geo Send & Init

* test=release/1.7,test=develop, cherry-pick 8f36c395

* add half_async in the communicator
* fix DistributedStrategy

* test=develop, bug fix for sparse recorder

* add distributed_strategy

* test=develop, speed dense calc & communication

* add fake init for the trainer, fix large memory hold in the trainer
* do not merge recv vars from a remote endpoint, test=develop
* add recv and save op, merge slice var in one op, save memory
* remove hsigmoid with pull sparse, test=develop

test=develop

* fix -Wno-error=sign-compare warning in gcc8

test=develop

* fix warning in distributed codes

test=develop

* remove -Wno-error=sign-compare, make warning as error

test=develop test=document_fix

* fix exist compile warning

test=develop

* test=develop,fix rpc not wait in geo

* update pserver decay blocks

* update distributed notify handler

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

* test=develop, communicator_is_sgd_optimizer flags

* 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

* bug fix: invalid learning rate decay in pserver async mode

* delete debug vlog & add rpc function & fix word2vec bug & speed GEO-SGD

add executor.FetchHandler for train/infer from the dataset

Heartbeat for distributed async training.

* refector geo sgd & communicator

* add a base class for the Communicator
* add AsyncCommunicator Impl for async distributed training

* rpc retry for asycsend/get/prefetch

* test=develop, change retry vlog level to 3

* test=develop, set default grpc_retry_times is 3

* remove -Wmaybe-uninitialized warning

test=develop

* remove uninitialized op_handle_ in scale_loss_grad_op_handle.cc

test=develop

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

* fix correctness of the communicator

* fix a bug in send thread when sending var context is empty, test=develop

* add lookup_table_prefetch_op and prefetch optimize, test=develop

* remove remote prefetch GPU supported

* word2vec force with CPU, test=develop

* test dist remote lookup table force with CPU, test=develop

Unset unittests http_proxy env to avoid timeout.

Polish fleet API to support cuda collective mode and nccl2 mode

* add is_runnning in communicator, test=develop

* optimize communicator merge sparse gradient test=develop

* revert multithread selected rows merge add test=develop

* follow comment test=develop

* fix save/load in Fleet
* add UT framework of Fleet

* remove attribute in Allocator::Allocate, test=develop

* fix travis ci error, test=develop