Add yaml and unittest for bmm op

* 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 gradient check to reduce ops

* add skip gradient check to reduce ops test=develop

* modify stack api test=develop

* add bmm op and move serval ops from fluid.layers to tensor test=develop