[dtensor] add StrategyType and TupleStrategy (#99435)
This PR refactors the current StrategyList. It introduces a
StrategyType, which is the base class of Strategy, and it have
two sub strategies:
1. Refactor the previous StrategyList to OpStrategy
2. Add TupleStrategy, the new strategy added to deal with tuple cases where
it could return multiple different OpStrategy for an op.
This would help support a more complicated op and unblocks compile mode
FSDP
Pull Request resolved: https://github.com/pytorch/pytorch/pull/99435
Approved by: https://github.com/mrshenli
