* 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

* test=develop
add sign op

* Update nn.py

test=develop
delete stop_gradient assignment

* 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

This reverts commit f456c8be.

* 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
test=develop

* 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.

* rename the default version from '0.0.0' to 'latest'

* test=develop
add elementwise_mod and elementwise_floordiv, fix equation problem in elementwise_mod

* fix some logic in distributed transpiler, test=develop
* reformat fleet API, test=develop

The parameter dirpath will be passed directly to c++ operater. The file address format will be different under win and UNIX.

* fix train_from_dataset and infer_from_dataset example

* add inductive dim for data_reader, example: shape=[-1, 1], then -1 will be inducted through run-time reading of number of elements

* Fix API example code of save_inference_model

test=develop

* Add "import" in exmaple of save_inference_model

* Fix typo "exsample" -> "example"

test=develop

* modified formula for lrn

test=develop

* modified api.spec

test=develop

Fix the following API examples:

paddle.fluid.scope_guard
paddle.fluid.backward.append_backward
paddle.fluid.cpu_places
paddle.fluid.cuda_pinned_places
paddle.fluid.cuda_places
paddle.fluid.in_dygraph_mode
paddle.fluid.CUDAPlace
paddle.fluid.CPUPlace
paddle.fluid.CUDAPinnedPlace

* resolve #17184

Fix the KL algorithm bug when calculated the size of tensor.

* Add the missing test=develop.

test=develop

* fix API examples, test=develop

* added lrn op test=develop

* Added CreateConstant method test=develop

* avoid duplicates test=develop

* polish doc
test=develop

* updata parallel executor doc
test=develop

* update API.spec
test=develop

* polish code
test=develop

* Fix data and reader related api doc

* Fix data and reader related api doc

Review and fix the example code in some reader related API doc.

These APIs are:

Fix existing API example codes:

paddle.fluid.io.PyReader
paddle.fluid.layers.batch
paddle.fluid.layers.data
paddle.fluid.layers.Preprocessor
paddle.fluid.layers.py_reader
paddle.fluid.program_guard

Add new example codes:

paddle.fluid.io.PyReader.decorate_batch_generator
paddle.fluid.io.PyReader.decorate_sample_generator
paddle.fluid.io.PyReader.decorate_sample_list_generator
paddle.fluid.io.PyReader.reset
paddle.fluid.io.PyReader.start

test=develop

* Add changes to API.spec after changing doc.

test=develop

* Add blanks after python example code

test=develop

* Add blank line at py_reader example code

test=develop

* Merge API.spec

test=develop

* Modify reader.py based on reviewer's comment

test=develop

* Modify API.spec after changing doc

test=develop

* Change reader.py based on reviewer's comment

* Modify example code of decorate_sample_generator

test=develop

* Fix example code of PyReader based on reviewer

test=develop

* add use_cuda to inplace pass,test=develop

* add test softmax_with_xe_inplace test,test=develop

* fix potential inplace bug
test=develop

* add more skip vars in mem opt pass,test=develop

* follow comment,test=develop

* follow comments,move duplicate out arg check to program->graph,test=develop

test=develop

* fix api doc of hash, relu, concat, argmin, argmax, argsoft and all activations funcs with no attrs

test=develop

* refine doc example code

test=develop

* remove >>> in doc example

test=develop

* refine python code block

test=develop

* update API spec

test=develop

* remove unused FLAGS_warpctc_dir

test=develop

* remove FLAGS_warpctc_dir

test=develop

* add attr axis infershape. test=develop

* add CUDA kernel. test=develop

* fix unittest. test=develop

* fix unittest for soft_label. test=develop

* fix fp16 unittest. test=develop

* remove comment code. test=develop

* refine test for axis. test=develop

* add python api. test=develop

* fix doc. test=develop

* fix fp16 unittest. test=develop

* fix ngraph test. test=develop

* fix ENFORCE for test_imperative_transformer. test=develop

* fit for ngraph test. test=develop

* fix after rebase develop. test=develop

* fix doc. test=develop

* fix API.spec. test=develop

* fix test_layers. test=develop

* fix format. test=develop

* Add MovingAverageAbsMaxScale operator which is only used for calculating the quantization scale.

* test=develop

* change the output into inplace. test=develop

* Revert "test=develop"

This reverts commit 696cf626.

* Revert "change the output into inplace. test=develop"

This reverts commit a19acd20.

* test=develop.

* update the MovingAverageAbsMaxScaleOp test. test=develop