[spmd] Introduce ParallelMode and add DTensorExpandMode (#98452)
This PR introduces a ParallelMode interface to define how to do
SPMD expansion and optimize the captured graph. This would be
beneifical for different parallelisms to expand differently
and apply different optimization passes
Put DTensorExpandMode as the first parallel mode that does the
existing dtensor_expand functionality.
Differential Revision: [D45174399](https://our.internmc.facebook.com/intern/diff/D45174399)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98452
Approved by: https://github.com/mrshenli
