* update execution_strategy option default value
test=develop

* fix doc error
test=develop

* remove nccl dep when the number of GPU is 1
test=develop

* update CPU_NUM config
test=develop

* fix the ParallelExecutor on Windows
test=develop
* restrict to use one GPU only under windows

test=develop

* add event for fast executor and add threads for scopebuffer executor
test=develop

* reset drop local scope counter
test=develop

* move pass to ir

* polish code
test=develop

* fix dependency
test=develop

integer', test=develop

test=develop

* modify the interface of Pass::Allay
test=develop

* Polish code
test=develop

* Fix Travis CI
test=develop

* fix Pass::Apply interface
test=develop

* Fix Travis CI
test=develop

* wip allreduce in op

* wip

* wip

* wip

* wip adding test

* wip for conflict with mp mode

* fix tests test=develop

* fix cpu build test=develop

* fix travis clang format test=develop

* fix cpu build test=develop

* update api.spec test=develop

* delete comment test=develop

* fix cpplint test=develop

* fix test=develop

* follow comment test=develop

* add file test=develop

* fix build test=develop

* update test=develop

* to be compatible with sync_bn, and fix mp mode in develop test=develop

* Support Sync Batch Norm.
* Note, do not enable it in one device.

Usage:

build_strategy = fluid.BuildStrategy()
build_strategy.sync_batch_norm = True
binary = fluid.compiler.CompiledProgram(tp).with_data_parallel(
        loss_name=loss_mean.name,
        build_strategy=build_strategy)

* fix broadcast with mp mode

* polish code test=develop

* fix bcast strategy test=develop

* fic cpplint test=develop

* fix py3 failed test=develop

* fix comment test=develop

* update comment test=develop

test=develop

test=develop

test=develop