* 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 strategy, test=develop

* fix can_apply

* consider the combination of different strategies to work together

* add check approval
test=develop

* move paddle.fleet to paddle.distributed.fleet

* refine strategy compiler and meta optimizers
make async as a_sync

refactor fleet api under paddle.fleet
update DistributedStrategy