* Add api doc code examples

add or fix topk, squeeze, stack, StaticRNN,
StaticRNN memory in doc

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

* Add squeeze md5.

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

* Add import package

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

* test=develop

* test=develop

* test=develop

* add collect fpn proposals op,test=develop

* test=develop
add sign op

* Update nn.py

test=develop
delete stop_gradient assignment

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

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

* modified formula for lrn

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* modified api.spec

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* fix API examples, 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

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* Add changes to API.spec after changing doc.

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* Add blanks after python example code

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* Add blank line at py_reader example code

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* Merge API.spec

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* Modify reader.py based on reviewer's comment

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* Modify API.spec after changing doc

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* Change reader.py based on reviewer's comment

* Modify example code of decorate_sample_generator

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* Fix example code of PyReader based on reviewer

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test=develop

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

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* refine doc example code

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* remove >>> in doc example

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* refine python code block

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* update API spec

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* 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

test=develop

* refine api comment, test=develop

* fix unexecutable API comments, test=develop

* add API.spec,test=develop

* fix distribute fpn proposals, test=develop

fix python3 run_time_error in layers.ops caused by locals()

fix runtimeerror : dictionary changed size during iteration when calling uniform_random in python3+

* refine_dropout_mem,test=develop

* # This is a combination of 14 commits.
# The first commit's message is:
remove ut test_dist_word2vec in mac ci, will fix it in private, test=develop (#17066)

# This is the 2nd commit message:

Fleet unify distributed training (#16791)

* implement distributed transpiler with fleet
# This is the 3rd commit message:

ParallelDyGraph with GPU collective mode (#16827)

implement dygraph.parallel.DataParallel to hook reduce op.

# This is the 4th commit message:

Init mixed precision training interface (#16856)

* Init mixed precision training interface

* Add fp16 test script

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* All initializers support float16

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* Code cleanup & add more code annotations

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* Update API spec

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* Add usage example in doc

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# This is the 5th commit message:

fix reference_count_pass,test=develop (#17060)

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# This is the 6th commit message:

Speedup roi_perspective_transform op by caching the information of linear interpolation in forward (#17090)

* Cache the information of linear interpolation in forward and use it in backward.
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* Fix cuda kernel.
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# This is the 7th commit message:

remove unnecessary prepare_data (#17080)

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# This is the 8th commit message:

fix interpolate cu. test=develop (#17101)

# This is the 9th commit message:

test=develop, double backward leaky_relu (#17067)

backward of backward: leaky_relu
# This is the 10th commit message:

fix fuse optimizer ops (#17102)

test=develop
# This is the 11th commit message:

truncated_gaussian_random supported in distributed training, test=develop (#17091)

# This is the 12th commit message:

 Detailed coordinate description for yolov3 loss (#17007)

* Detailed coordinate description for yolov3 loss

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* modified api.spec

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* modified loss name

* fix api.spec

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* polish description

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* modified api.spec

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# This is the 13th commit message:

fix test_weight_decay (#17109)

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# This is the 14th commit message:

Path flag (#17105)

* fix python/paddle/fluid/__init__.py detecting problems

* Detailed coordinate description for yolov3 loss

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* modified api.spec

test=develop

* modified loss name

* fix api.spec

test=develop

* polish description

test=develop

* modified api.spec

test=develop

Speedup roi_perspective_transform op by caching the information of linear interpolation in forward (#17090)

* Cache the information of linear interpolation in forward and use it in backward.
test=develop

* Fix cuda kernel.
test=develop

implement dygraph.parallel.DataParallel to hook reduce op.

test=develop

test=develop

* Check some shapes only in runtime

test=develop

* Follow review comments

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* Update API spec

* test=develop
update API.spec with hash, resolve conflict

* remove unused parameter

* optimize lstmp and sample_logits op, test=develop

* update op_use_default_grad_op_maker.spec, test=develop

* delete useless file,test=develop

* append 0 to dim variable to avoid memory reusage, test=develop

* test=develop, fix hsigmoid dereference nullptr

* test=develop, refine condition

* test=develop, refine comments

test=develop

* enable recurrent op test=develop