* add authors
Co-authored-by: NAllen Guo <alleng@graphcore.ai>
Co-authored-by: NZhixin Yao <zhixiny@graphcore.ai>
Co-authored-by: NZhaorui Chen <zhaoruic@graphcore.ai>

* squash py changes 0/N
Co-authored-by: NZhixin Yao <zhixiny@graphcore.ai>
Co-authored-by: NZhaorui Chen <zhaoruic@graphcore.ai>

* 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

* upgrade to sdk2.5.1

* update UTs 0

* fix ci

* fix ci 3

* add mixed-precission support for ipu

* restore cast_model_to_fp16 api

* update UTs

* update ipu UTs part0

* rename UT

* sync api changes

* update uts for new api

* use_ipumodel() as classmethod