* 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

* Rename `Input` into `InputSpec`

* remove alias

* add erf op and python interface.

* add fp16 support for erf op.

* add unitests for erf op and its python interface.

    Add a python interface for Gelu.
    Add documentation for fluid.layers.gelu.