* add documentation for DistributedStrategy

* consider the combination of different strategies to work together

* add save/load for parameter server

* add feature to fleet2.0 role_maker, distribute_strategy, test=develop

* add check approval
test=develop

* make fleet_localsgd_meta_optimizer work

* fix bug in localsgd meta optimizer

* set default value to strategy in distributed_optimizer test=develop

* add unittest for paddlerolemaker with gloo

* move paddle.fleet to paddle.distributed.fleet

test=develop

* fix the compatibility of PY2 and PY3 in paddle.distributed.launch
test=develop

* only pull log of local rank 0
test=develop

* log exception if UnicodeEncodeError occurs when pulling log in
paddle.distributed.launch
test=develop
Co-authored-by: NSunGaofeng <peakbee@gmail.com>

test=develop

replace join to terminate

* add DataLoader, Dataset, BatchSampler

* add launch_ps module so that we can launch a parameter server training job
1) a user can specify worker_num and server_num
2) parameter server can be killed after all workers exit
3) unit test is added
test=develop

distribute.launch use poll to query subprocess

distribute launch : add use_paddlecloud argument 

node_num is not needed for users, so remove them and fix the bugs about it!

refine launch_ps and role_maker

add parameter server launch so that a user can easily launch parameter server

test=develop

add NCCLParallelContext for parallel dygraph

* add launch mp distributed mode module test=develop

* delete unused file test=develop

* refine usage test=develop

* refine usage test=develop

* move distributed package test=develop

* add to whl package test=develop