* 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

* Update __init__.py

* Update math.py

* Create test_nanmean_api.py

* Update __init__.py

* Update __init__.py

* Update math.py

* Update test_nanmean_api.py

* Update __init__.py

* Update math.py

* Update test_nanmean_api.py

* Update test_nanmean_api.py

* Update test_nanmean_api.py

* Update math.py

* Update test_nanmean_api.py

* Update math.py

Update the nanmean example code

* Update math.py

* Update math.py

* Update math.py

Remove redundant code in nanmean

* Update math.py

change default keepdim = False

* Update test_nanmean_api.py

add nan into self.x

* Update test_nanmean_api.py

rerun CI check

* Update test_nanmean_api.py

* update code of nanmean in python/paddle/tensor/math.py and test_nanmean_api.py

* Update test_nanmean_api.py

update code format

* Update test_nanmean_api.py

update code format

* Update test_nanmean_api.py

add check grad code.

* Update math.py

update nanmean's describe of Args x

* Update test_nanmean_api.py

update format and release the test_case(self.x, keepdim=True) in check grad code.

* Update test_nanmean_api.py

Update gradient checking method

* Update test_nanmean_api.py

update code format

* Update test_nanmean_api.py 

Update code format and copyright in test_nanmean_api.py

* Update math.py

update arguments describe and code example

* Update math.py

修改了nanmean的axis参数的文档描述。

* Update math.py

updata nanmean's sample code (:name: code-example1)

* Update math.py

修改nanmean的example code 错误

* Update math.py

update example code

* Update math.py

update example code of nanmean