[IPU] Decoupling ipu sharding and modeling

[gglin001]

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文档对应的 PR PaddlePaddle/docs#4855
这个PR是 PR #42961 的重复, 因为 #42961 提示

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[qili93]

LGTM

[gglin001]

补充信息:
这里的 CI PR-CI-iScan-Python 报错应当属于误报, 报错信息为:

Context manager 'generator' doesn't implement enter and exit.

对应代码为:

        def __call__(self, *args, **kwargs):
            with ipu_shard_guard(index=index, stage=stage):
                return super().__call__(*args, **kwargs)

ipu_shard_guard 对应的接口是

@signature_safe_contextmanager
def ipu_shard_guard(index=-1, stage=-1):
...

使用 @signature_safe_contextmanager 装饰会使得 ipu_shard_guard 具有 context manager 的性质

[Ligoml]

LGTM for docs