* add DenseNet

* add inceptionv3
Co-authored-by: Ainavo <ainavo@163.com>
Co-authored-by: Npithygit <pyg20200403@163.com>

* add resnext model
* add zh docs
* add unittest
* test performance
Co-authored-by: Ainavo <ainavo@163.com>
Co-authored-by: Npithygit <pyg20200403@163.com>
Co-authored-by: Ainavo <ainavo@163.com>
Co-authored-by: Npithygit <pyg20200403@163.com>

* add alexnet

* add roi_align in vision/ops.py

* add roi pool

* rename input as x

* add func/class API psroi_pool and UT

* add UT in static mode

* Remove redundant type checks in static mode

* More detailed description for test_psroi_pool_op

* fix code format of UT

* fix en-doc

* fix warning error , test=document_fix

* fix warning error , test=document_fix

* fix warning error , test=document_fix

* fix warning error , test=document_fix

* fix warning error , test=document_fix

* fix warning error , test=document_fix

* fix warning error , test=document_fix

* add input and output docs for vision transform

This reverts commit 6c110344.

* cache core.ops, test=develop

* refine, test=develop

* fix random probability

* fix random probability

* del py2 code2

* fix test timeout

* update 2.0 public api in vision

* fix some flake8 errors

* add two attributes for yolo box

* Fix slow data reading, In the old version, one epoch read time of this data set was about 5371 seconds(MacBook Pro Retina, 13-inch, Early 2015 2.7 GHz), and a batch took 211 seconds, It's too painful to use. Now decompress the data in advance (about 10 seconds). Each epoch of reading takes about 3 seconds(MacBook Pro Retina, 13-inch, Early 2015 2.7 GHz), and a batch takes 0.017 seconds more.

* Run CI, test=allcase

* fix qq group number. test=document_fix

 fix qq group number. test=document_fix

* fix qq group number. test=document_fix 

fix qq group number. test=document_fix

* add to_grayscale, normalize

* add rotate

* add vfip and hflip

* add crop center_crop


* add padding, support constant, reflect, replicate, circular same as paddle.pad

* add get-image-[n,c,w,h] axis utils

* add op read_file and decode_jpeg

* fixed docs.

* Fixed docs. test=document_fix

code bak.

fixed docs. test=document_fix

* Revert to previous version of python/paddle/fluid/backward.py

* fixed bugs.

* test=document_fix. Fixed examples.

* Support list and tuple for parameters of layer_norm, multiprocess_reader, DatasetFolder and ImageFolder.

* add unittest for layer_norm.

* add require gpu for example.

* Modified params of some APIs to support tuple and list.

* fixed bug.

* update comments

* add attrs deformable_groups

update RandomHorizontal Flip random description and examples

* fix test_datasets unittest

* update document of paddle.vision.dataset, test=document

* update document of paddle.vision.dataset, test=document

* update voc url

* fix cv2 rotation

* update to_tensor en docs

* add deform_conv2d,DeformConv2D

* Update Cifar Codes

* Update VOC2012 Codes

* Update voc2012.py

* Update voc2012.py

* Update cifar.py

* Update cifar.py

* Update voc2012.py