[dtensor] refactor and generalize stack strategy (#121869)
This PR rewrite the stack strategy to be more generalized, basically
stack/cat like strategy follow pattern need to be smarter, i.e. it
should be able to identify:
1. PR, PP, RP -> follow PP
2. RR, SR, RS -> follow SS
So this PR refactors how the follow strategy should work, and make sure
we start following the strategy that incurred lowest cost. i.e. for
multiple PR, RP placements, we should be able to further delay the
pending sum reductions
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121869
Approved by: https://github.com/awgu
