This reverts commit e680d581.

* [IPU] support paddle.distributed.launch with IPUs

* add device_num to env_args_mapping

* upgrade to sdk2.5.1

* update to popart v2.5.0

* use a specific version of sdk2.5.0

* update UTs 0

* fix ci

* fix ci 3

* merge recent changes

* fix setting pipline

* add mixed-precission support for ipu

* restore cast_model_to_fp16 api

* update UTs

* add ipu uts

* fix ut

* split PR

* fix ut

* rm ut

* add more ops

* add authors
Co-authored-by: NXiaobing Wang <xiaobingw@graphcore.ai>
Co-authored-by: NAllen Guo <alleng@graphcore.ai>
Co-authored-by: NZhixin Yao <zhixiny@graphcore.ai>
Co-authored-by: NZhaorui Chen <zhaoruic@graphcore.ai>
Co-authored-by: NHan Zhao <hanzhao@graphcore.ai>

* rm ipu_strategy.check()

* fix UT fail

* fix typo
Co-authored-by: NXiaobing Wang <xiaobingw@graphcore.ai>
Co-authored-by: NZhixin Yao <zhixiny@graphcore.ai>
Co-authored-by: NZhaorui Chen <zhaoruic@graphcore.ai>
Co-authored-by: NHan Zhao <hanzhao@graphcore.ai>

* sync changes

* copy sOpNamescope

* fix UTs

* add authors
Co-authored-by: NXiaobing Wang <xiaobingw@graphcore.ai>
Co-authored-by: NAllen Guo <alleng@graphcore.ai>
Co-authored-by: NZhixin Yao <zhixiny@graphcore.ai>
Co-authored-by: NZhaorui Chen <zhaoruic@graphcore.ai>
Co-authored-by: NHan Zhao <hanzhao@graphcore.ai>

* fix code-format

* fix compile error

* add comments for feed_op
Co-authored-by: NXiaobing Wang <xiaobingw@graphcore.ai>
Co-authored-by: NZhixin Yao <zhixiny@graphcore.ai>
Co-authored-by: NZhaorui Chen <zhaoruic@graphcore.ai>
Co-authored-by: NHan Zhao <hanzhao@graphcore.ai>

* add ci

* rm retry tests

* format

* restore retry tests

* update timeout for ipu uts

* update ipu UTs part1

* rename ut

* sync api changes

* update uts for new api

* update use_ipumodel()

* update use_ipumodel()

* split pr

* update ipu UTs part3

* rename uts

* sync api changes

* update uts for new api

* update use_ipumodel()

* split pr

* update ipu UTs part4

* rename uts

* sync api changes

* update uts for new api

* update ipu UTs part2

* clean git

* rename ut

* rename ut 1

* sync api changes

* update uts for new api

* update uts for new api

* fix re-define

* update ipu UTs part0

* rename UT

* sync api changes

* update uts for new api

* use_ipumodel() as classmethod