* 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

* add expand, poisson

* add poison grad

* add expand equal_all poisson triangular solve yaml

* change CUDA implementaion of bernoulli OP

* fix CI

* add new API/OP:paddle.poisson

* fix comment