* some fixes for int8 mobilenet_ssd tester
test=develop

* change wrong data file name
test=develop

* change test images bin file from 200 images to 100 images

* change directory existence to file existence during downloading
test=develop

* reuse download_data
test=develop

* run full dataset when iterations=0
test=develop

* - Reusing of reuder used in elementwise_add_mkldnn

- Added MKL-DNN sum prim reusing

test=develop

- Compilation fixes

test=develop

- Yet another compilation fix

test=develop

- Yet another compilation fix

test=develo

- Yet another linking fix

test=develop

- Final compilation fix

test=develop

- lint fixes

test=develop

- Lint fixes

test=develop

* - Fixes after review

test=develop

* Simplify multi_box_head API in detection.py and remove assign op.

* refine cuda place error msg, test=develop

* use LOG(ERROR)+exit(-1), test=develop

test=develop

* Update lamb optimizer

test=develop, test=document_preview

* Regenerate api spec

test=develop, test=document_preview

test=develop

Socket can't be pickled.
test=develop

* fix softrelu doc

test=develop

* update API doc

test=develop

* test=develop, add_install_check_for_multigpu

* test=develop, refine code to use cuda_devices

* sequnce mask support max length tensor input; test=develop

* add rnn_impl.py; test=develop

* add basic gru lstm unittest; test=develop

* fix api spec; test=develop

* fix sequence_mask op bug;
test=develop
test=document_preview

* change +-*x to elmentwise_op; test=develop

* add mkl flag; test=develop

* fix rnn impl bug; test=develop

* update api spec; test=develop

* fix doc bug; test=develop

* fix lstm bugs; test=develop

* Revert "Add multi gpu install check (#18229)"

This reverts commit 61ed06b2.

* test=develop, start ci

* optimize communicator merge sparse gradient test=develop

* revert multithread selected rows merge add test=develop

* follow comment test=develop

test=develop

* fix default value of fluid.memory_optimize
test=develop

* fix api.spec
test=develop

test=develop

* Use TensorCopySync for sequence_unpad op

test=develop

* Fix the tensor memory alloc bug

test=develop

* clean build_strategy
test=develop

* DataBalanceOpHandle has been removed
test=develop

* debug

* update build_strategy.
test=develop

test=develop

add Dygraph api to api.spec

* improve doc of lstm, sequence_enumerate, softmax_with_cross_entropy, space_to_depth APIs, test=develop

* update API.spec, test=develop

test=develop

* fix paddle cloud role maker bug

test=develop, test=document_preview

test=develop
test=document_preview

* refine core cmake warning and print more info

test=develop

* fix comments

test=develop

refine StaticRNN api doc
test=develop
test=document_preview

1. fix the bug that out_put_var in SaveSelectedRows would be empty string
2. use merge_sparse_lookup_table to replace sum op for load_persistables_for_inference
3. fix the bug in _clone_var_in_block_ when the var is SELECTED_ROWS.

(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