(1) use channel instead of vector/BlockingQueue in Dataset，to keep same with existing implementation, and make code more readable and flexible (dataset single output channel or multi output channel). one previous memory out of limit problem is cause by not release memory after training.
(2) add Record because MultiSlotType costs too much memory (80B)，fix memory out of limit problem.
(3) add Channel, Archive in paddle/fluid/framework
(4) change dataset from shared_ptr to unique_ptr in pybind
(5) move create/destroy readers from trainer to dataset
(6) move shuffle from datafeed to dataset. dataset holds memory, datafeed is only for load data and feed data to network.
(7) fix thread num bug of Dataset when filelist size < thread num
(8) support set_queue_num in InMemoryDataset

* improve the doc of DataFeeder and default_main_program

* update API.spec, test=develop

* set src_idx > 0, test=develop

* add unittest and cu, test=develop

* [DOC] Fix comment code of API create_py_reader_by_data. test=develop, test=document_preview

* Fix code style of API comment. test=develop,test=document_preview

Fix code style of API comment. test=develop,test=document_preview

* update api spec of api create_py_reader_by_data

* remove default config code. test=develop

* remove useless code. test=develop

* update create_py_reader_by_data api. test=develop

* fix slice op bug; test=develop

* fix variabel test bug; test=develop

* remove slice while true; test=develop

* fix paddle.fluid.layers.io.open_files api doc bug test=develop

* add multi process reader
test=develop

* API.spec test=develop

* update

* update test=develop

* update test=develop

* update

* update test=develop

* update test=develop

* update test=develop

* update test=develop

* update test=develop

* test=develop

* update

* update test=develop

* update test=develop

* fix test=develop

* fix issue 18111;test=develop

* fix timer;test=develop

* refine code;test=develop

* fix spelling errors; test=develop

* Update API.spec

update md5

* Update API.spec

* change the order of api;test=develop

Fix doc example for greater_equal, greater_than, less_equal, not_equal, rank, reduce_all, reduce_any, sign, where, diag (#18167)

* test=develop
fix greater_than, greater_equal, less_equal, not_equal, rank, reduce_all, reduce_any, sign, where, diag doc example

* test=develop
fix API.spec conflict

The fixed APIs:
6 Methods in paddle.fluid.io.PyReader
paddle.fluid.layers.Preprocessor
paddle.fluid.layers.py_reader
paddle.fluid.io.save_params
paddle.fluid.io.save_persistables

test=develop
test=document_preview

* 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

* fix py_reader iterable bug, test=develop

* move data from buffered_reader,test=develop

* 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

* 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

* fix code bug
test=develop

As title

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

* 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

* added unit test for QAT FP32 & INT8 comparison

test=develop

* enabled other models and updated filenames

test=develop

* added accuracy check and multiple batch handling

test=develop

* removed quantization_mkldnn_pass.py

test=develop

* cleanup

test=develop

* updated model paths

test=develop

* renamed tests without MKL-DNN

test=develop

* fix reusing mkldnn pool2d primitive

test=develop

* add performance measuring

test=develop

* fix accuracy statistics

test=develop

* removed non-mkldnn tests

test=develop

* added conv2d_depthwise->conv2d mkldnn transformation

test=develop

* format update

test=develop

* fixed creating key for pool2d grad

test=develop

* added pass

* Fix the accuracy issue while using float precision to get the scale.

test=develop

* Fix the format issue when 'X' is not nchw.

test=develop

* removed output comparing and changed number of images

test=develop

* cmake and comment fix

test=develop

* updated acc threshold for QAT comparison tests

test=develop

* added OMP_NUM_THREADS setting

test=develop

* enable all QAT INT8 tests

test=develop

* restored upstream version of a file

test=develop

* modified directory names

test=develop