* 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

* refactor eager flags

* fix flags error when we switch from eager to dygraph

* fix ci problem

* fix ci

* fix ci

* merge develop and fix code style

* merge develop and fix code style

* fix op test error

* fix op test error

* fix op test error

* fix op test error

* fix op test error

* merge develop

* fix drop_last not work in IterableDataset. test=develop

* add persistent_workers. test=develop

* del py2 code2

* fix test timeout

* add random state generate in DataLoader worker. test=develop

* polish tensor pipeline. test=develop

* add dict/str/list supprot for DataLoader. test=develop