* 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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* 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

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* refine api comment, test=develop

* fix unexecutable API comments, test=develop

* add API.spec,test=develop

* fix profiler and name_scope API examples

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* update API.spec

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* remove async executor python api
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* remove test_async_executor.py
add executor train_from_dataset demo
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* fix import bug
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* 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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* 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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* Check some shapes only in runtime

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* Follow review comments

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* Update API spec

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

* remove unused parameter

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* enable recurrent op test=develop

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* fix some comments, include cosine_decay,l2_normalize,pixel_shuffle

* Add api.spec, test=develop

* update api.spec, test=develop

* add api.spec,test=develop

* test=develop

* test=develop

* fix conflict,test=develop

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* add pixel_shuffle op

* add pixel_shuffle op, test=develop

* rewrite code, test=develop

* delete useless comment, test=develop

* Refine pixel_shuffle_op and unit testing

* refine code,test=develop

* refine .cu,test=develop

* fix unittest,test=develop

* Fix unit testing
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* resolve conflict, test=develop

* fix test, test=develop

* fix API, test=develop

* fix test datatype bug,test=develop

* polish comments,test=develop

* add API,test=develop

* test=develop

* Add Pixel_Shuffle OP,test=develop

* support python3,test=develop

* add include memory to travis CI bug,test=develop

* Add linear learning warmup method

This warmup lr can be combinated with other learning rate strategies.
For example:
            decayed_lr = fluid.layers.linear_lr_warmup(
                fluid.layers.piecewise_decay(boundaries, lr_steps),
                warmup_steps, start_lr, end_lr)

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* Add DecoupledWeightDecay

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