* Fix doc format for callbacks, metrics and Model
* Fix code sample and doc

* upgrade comment string to raw string

* fix string in

* fix string with ' '

* revert update on comments

* upgrade only necessary

* fix sample code checker

* fix comments with '''

* add WeightedRandomSampler. test=develop

* DataLoader support not auto collate batch. test=develop

* add paddle.io.ComposeDataset & paddle.io.ChainDataset. test=develop

* fix DataLoader return same format between static & dynamic in single mode. test=develop

* fix dataloader

* fix DataLoader single process mode exit SIGABRT error. test=develop

* move worker loop to top level

* move reader process loop to top level

* fix failed unittests

* 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

* add TensorDataset. test=develop

* fix RandomSampler & BatchSampler. test=develop

* add Sampler/SequenceSampler/RandomSampler. test=develop

* fix test_multiprocess_dataloader unittest. test=develop

* add IterableDataset support in multiprocess DataLoader. test=develop

* add pin memory control

* fix buffered reader init problem

* fix unittest error

* add unittest for coverage

* make default_collate_fn visible. test=develop

* fix test_multiprocess_dataloader_static random fail. test=develop

* add DataLoader, Dataset, BatchSampler