* update 2.0 public api in vision

* fix some flake8 errors

* 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

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

* add unittest for layer_norm.

* add require gpu for example.

* fix test_datasets unittest

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

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

* update voc url

* Update Cifar Codes

* Update VOC2012 Codes

* Update voc2012.py

* Update voc2012.py

* Update cifar.py

* Update cifar.py

* Update voc2012.py

* lazy import for scipy

* rm unused check

* return pil image as default according backend

* add pil backend

* fix CIFAR & MNIST dataset. test=develop

* rm dependence of cv2

* Move hapi form paddle/incubate to paddle

* Remove vision/datasets/utils.py and clean code

* Add sample code for conll05

* Print pull path when saving model

* Fix sample code after paramter_list of SGD is changed to parameters

* Fix bug in wmt16 datase