This reverts commit aca60e9a.

Refine the Executor when the num_thread=1

* init auto loss scaling

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* change API.spec

* change ifelse to switch and use reduce_sum to optimize checking isfinite

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* Remove redundant code

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* add set_not_owned function for graph

* add scope set. test=develop

* add scope_ptr enforce not null before setting.test=develop

fix secure bug

* add save/load model, shrink table, cvm, config file & fix pull dense bug
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* fix global shuffle bug, fix pull dense bug, fix release memeory bug, fix shrink error
add client flush, add get data size
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* fix global shuffle bug
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* fix global shuffle bug
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* fix code style
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* fix code style & modify pslib cmake
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* fix error of _role_maker
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* fix code style
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* fix code style
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* fix code style
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* fix code style
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* fix code style
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* fix windows compile error of fleet
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* fix global shuffle bug

* add comment
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* update pslib.cmake
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* fix fill sparse bug
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* fix push sparse bug
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* double backward sqrt

* refine unittest. test=develop

* refine test. test=develop

* remove alpha in unittest. test=develop

* test=develop

* test=develop

* test=develop

* test=develop, double backward reduce_mean

* add comment. test=develop

* fix format. test=develop

* rename GradGrad -> DoubleGrad. test=develop

* fix op_use_default_grad_op_maker.spec. test=develop

* add elementwise_add_grad_grad op. test=develop

* use defined GradMaker. test=develop

* add collect fpn proposals op,test=develop

* add double grad for mul_op. test=develop

* fix format. test=develop

* fix format. test=develop

* fix format. test=develop

* refine code. test=develop

* remove setzero. test=develop

* fix dx/dy init bug. test=develop

* fix format. test=develop

Fix some api example codes' bugs and these APIs include load_inference_model, load_vars, save_vars, L1DecayRegularizer and L2DecayRegularizer. (#17324)

* fix some api example codes' bugs.

* update API.spec. test=develop test=document_preview

* add import fluid. test=develop test=document_preview

* make parallel_executor support FLAGS_use_mkldnn

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* add warning when set mkldnn_enabled_op_types_ in non-mkldnn env

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* test=develop
add sign op

* Update nn.py

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delete stop_gradient assignment

* Optimize the computing kernel of sequence_reverse operator.

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* Clean code

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* Fix for cpplint syntax checking.

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* Fix the compile warning issue.

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* Optimize the elementwise op with CUDA kernels.
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* Support setting of attr in op config file.
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* Add the support the setting dtype and initializer in config.
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* Save workspace.

* Add initializer "zeros".
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* Fix compiling error.

* Support the use of existed file to initailize tensor in op_tester.

* Use eigen to optimize the elementwise_add/mul for the case that x and y have the same dims.
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* test=develop, add gradient sort backward strategy

* test=develop, fix test by add FLAGS_cudnn_deterministic on new tests

* Fix the example code in some Python API
* Update paddle/fluid/API.spec
* Fix some examples format

* add double grad for elementwise_mul. test=develop

* remove comment. test=develop

* fix grad sum. test=develop

* fix for axis expand. test=develop

* add test for axis expand. test=develop

* add double grad for square. test=develop

* formax code. test=develop

* fix for grad sum. test=develop

* refine shape. test=develop

* refine extract. test=develop

* test=develop, test=document_preview, fix all 13 api doc and code

* test=develop, fix rst

* test=develop, refresh API.spec

* reset drop local scope counter
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refine code

fuse cublas  calling and kernels into one cuda kernel.

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Signed-off-by: Nzhaoyuchen <zhaoyuchen01@baidu.com>

* Add conv2d_grad_grad_op
* Extracte the cuDNN conv algo searching code in conv_cudnn_helper.h.
    - Now use it in conv2d_grad_grad.
    - Will simply the searching code in conv2d and conv2d_grad in next PR.
* Enhance and fix bug in unit testing of gradient_checker.
* Support to fetch empty variables，return None in Python.

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1. infernce multi card occupy
2. facebox model inference occupy too much
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