* update some op doc, test=develop

* test=develop
fix type error of std::pow in sigmoid_focal_loss_op.cu and sigmoid_focal_loss_op.h

* test=develop
fix wrong code stype in sigmoid_focal_loss_op.cu and sigmoid_focal_loss_op.h

* 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

* fix dygraph mem leak, test=develop

* polish msg, test=develop

test=develop

fix role_maker bug
test=develop

* fix py_reader iterable bug, test=develop

* move data from buffered_reader,test=develop

add paddle cloud role maker for customized usage, note this is only for industrial users that have cloud environment pre-configuration (#18121)

add paddle cloud role maker for specific cloud usage. This pr will simplifies user's configuration in distributed training.

* Update backward.py:
     - If there is no input grad var in all outputs of previous ops, do not append this op into graph.
     - Only apply this stragety when double backward.
* Update some double backward op.
* Update sum_op to judge whether a tensor is empty by numel or IsInitialized().

test=develop

* reuse C-API INT8 unit test application

test=develop

* updates after review

test=develop

* test=develop
add detection output for supporting retinanet

* test=develop
add test_layers.py

* test=develop
add API.spec

* test=develop
alter test_retinanet_detection_output.py

* test=develop
alter round 2

* test=develop
alter retinanet_detection_output

* test=develop
alter paddle/fluid/API.spec

* test=devlop
alter detection.py

* test=develop
alter retinanet_detection_output

* test=develop
alter paddle/fluid/API.spec

* test=develop
alter detection.py

* test=develop
alter API.spec

* test=develop
alter retinanet_detection_output

* test=develop
alter paddle/fluid/API.spec

* test=develop
alter python/paddle/fluid/tests/unittests/test_retinanet_detection_output.py

* test=develop
alter python/paddle/fluid/tests/unittests/test_retinanet_detection_output.py

* test=develop
fix grammer error

* test=develop
fix grammer error

* test=develop
fix grammer error

* test=develop
alter python/paddle/fluid/tests/unittests/test_layers.py

* test=develop
alter paddle/fluid/API.spec

* add class name and timeline for test_dist_base.py

update config for matplotlib, because python 3.6 is required by 3.1+

* test=develop
add sigmoid_focal_loss for supporting retinanet

* test=develop
add test_layers

* test=develop
add API.spc

* test=develop
alter sigmoid_focal_loss_op.cc

* test=develop
alter detection.py

* test=develop
alter API.spec

* test=develop
alter round 1

* test=develop
alter simooid_focal_loss

* test=develop
alter sigmoid_focal_loss_op.cc

* test=develop
alter test_layers.py

* test=develop
alter paddle/fluid/API.spec

* test=develop
alter sigmoid_focal_loss_op.cu

* test=develop
alter paddle/fluid/operators/detection/sigmoid_focal_loss_op.cc

* Update generate_proposal_labels_op to support CascadeRCNN.

* test=develop add target assign for retinanet

* test=develop
run ci

* test=developp
add test_layers

* test=develop
add APi.spec

* test=develop
alter round 1

* test=develop
alter rpn_target_assign_op.cc

* test=develop
alter test_rpn_target_assign_op.py

* test=develop
alter rpn_target_assign_op.cc

* test=develop

alter API.spec

* test=develop
alter paddle/fluid/operators/detection/rpn_target_assign_op.cc

* test=develop
alter rpn_target_assign_op.cc

* test=develop
alter python/paddle/fluid/layers/detection.py

* test=develop
alter paddle/fluid/API.spec

Jian Tang made change on latest-dev Dockerfile, so sync the change
in the cuda9/10 Dockerfile

test=develop

for py2:
matplotlib<=2.2.3 ; python_version<"3.5"
scipy>=0.19.0, <=1.2.1 ; python_version<"3.5"
nltk>=3.2.2, <=3.4 ; python_version<"3.5"

for py3:
matplotlib ; python_version>="3.5"
scipy ; python_version>="3.5"
nltk ; python_version>="3.5"

update requests version to >=2.20.0
for CEV-2018-18074

add luotao to approval requirements.txt

* fix code bug
test=develop

As title

test=develop

* fix get items slice bug; test=develop

* fix unique_name bug; test=develop

* add print log for unittest of distributed training
test=develop

test=develop

* Remove layers.detection_map API
* Since uers can use fluid.metrics.DetectionMAP to calculate mAP of current-batch and cumulative-batch. layers.detection_map only can calculate cur-batch mAP.

test=develop

* add pascalvoc preprocess script and mobilenet-ssd analyzer_tester, wait 17737

* change converting local dataset to downloading and converting tarfile
test=develop

* change the test data_path
test=develop

* change copyright (c) 2016 to copyright (c) 2019
test=develop

test=develop

* update CPU_NUM config
test=develop

* refractor the function ConvFwdPrimitiveDesc
test=develop

* change according to review
test=develop

* use pointer way without boost::optional
test=develop

* pass vector to function by reference instead of raw vector
test=develop

* change pointer to shared_ptr
test=develop