* [CodeStyle] remove all future import

* revert test_error.py

* restore future import in example code

* fix the bug that _DataLoaderIterMultiProcess use time to generate the seed

* use np.random.randint to generate a base seed

* use yapf to format all python file

* yapf exclude two unittests file for they rely on writing and reading file, and format will break them

* disable diff_py_file because too many diff files cause command following failed

* fix drop_last not work in IterableDataset. test=develop

* remove BatchSampler type check. test=develop

* DataLoader support not auto collate batch. test=develop

* move worker loop to top level

* move reader process loop to top level

* fix failed unittests

* add IterableDataset support in multiprocess DataLoader. test=develop

* simplify buffered reader to improve DataLoader performance

* fix 22 failed unittests

* fix cuda pinned context condition

* fix test_reader_reset failed

* fix two failed unittests

* change unittest place

* polish error messaage

* polish cast op GetExpecctedKernelType

* remove debug info in unittest

* fix test_multiprocess_dataloader_exception failed on CPU only version. test=develop

* add DataLoader, Dataset, BatchSampler