feat(hydroflow): [Tracking Issue] Hydroflow engine upgrade to support flo

[MingweiSamuel]

Driven by hydroflow+ / flo-semantics requirements

• [x] #1585 Add flo syntax/syntax checks (#1507 #1513)
• [x] #1580
  • [x] #1692
  • [x] #1612 / #1725
  • [x] #1731 / #1725
  • [ ] #1730
• [x] (#1618)
• [ ] Examples (k-means, transitive closure w/ shortest paths, for stratification)
• [ ] #1581

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• [ ] #1699
• [ ] #1697

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• [x] feature: https://github.com/hydro-project/hydro/issues/1622
• [ ] Performance: Optimizations for loop contexts as a DAG (ref handoffs)
• [ ] Performance: Specialize/optimize codegen/handoffs to pipeline flags (prev #166)
• [ ] Performance: Upgrade scheduler/codegen (based on DAG) for short circuiting of upstream branches
• [ ] #1670 (DAG subgraphs makes this issue easier)

(docs)

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• [ ] ~~#1672~~
• [ ] ~~#1653~~

[MingweiSamuel]

Scheduler upgrade plan:

• Strata do not matter in loops (?)
• We can start running a nested loop whenever, however…
  • Once we start running a loop we can no longer run the outer loop until the current loop is done (heuristic?)
  • Heuristic: just always run the deepest (most loop nestings) subgraph scheduled…?
• Loop state must be reset at some point after exiting the loop and entering it for a new iteration.