* Decoupling ipu sharding and modeling (#665)

* feat(shard): decoupling shard setting with modeling.

* fix(shard): split test cases to avoid failure.

* fix(shard): add function docs and fix typo.

* test(shard): add tests.

* test(shard): more test case.

* fix(): change ipu_index/stage default value to -1.

* fix format
Co-authored-by: Nczr-gc <96037699+czr-gc@users.noreply.github.com>