Expose Sabre heuristic configuration to Python (backport #12171)

[mergify]

Summary

This exposes the entirety of the configuration of the Sabre heuristic to Python space, making it modifiable without recompilation. This includes some additional configuration options that were not previously easily modifiable, even with recompilation:

• the base weight of the "basic" component can be adjusted
• the weight of the "basic" and "lookahead" components can be adjusted to either use a constant weight (previously not a thing) or use a weight that scales with the size of the set (previously the only option).
• the "decay" component is now entirely separated from the "lookahead" component, so in theory you can now have a decay without a lookahead.

This introduces a tracking Vec that stores the scores of all the swaps encountered, rather than just dynamically keeping hold of the best swaps. This has a couple of benefits:

• with the new dynamic structure for heuristics, this is rather more efficient because each heuristic component can be calculated in separate loops over the swaps, and we don't have to branch within the innermost loop.
• it makes it possible in the future to try things like assigning probabilities to each swap and randomly choosing from all of them, not just the best swaps. This is something I've actively wanted to try for quite some time.

The default heuristics in the transpiler-pass creators for the basic, lookahead and decay strings are set to represent the same heuristics as before, and this commit is entirely RNG compatible with its predecessor (technically for huge problems there's a possiblity that pulling out some divisions into multiplications by reciprocals will affect the floating-point maths enough to modify the swap selection).

Details and comments

I need to run more formal benchmarks on this to verify the claims in the commit message - it's possible I didn't do it sufficiently well earlier, and at any rate I no longer have the data to hand. (edit: come to think of it, I'm not even sure I ran the tests...)

I so far have not documented the newly available API stuff from Rust-space here, because I didn't necessarily want to commit to it as public interface, but did want to make it available for research - certainly I found this very useful in investigating Sabre, even if I haven't had time to turn that research into a full set of tweaked heuristics yet or anything. Happy to discuss whether we should commit to this as public API - there's probably not too much risk.

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This is an automatic backport of pull request #12171 done by Mergify.

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[coveralls]

Pull Request Test Coverage Report for Build 10165244742

Details

• 135 of 240 (56.25%) changed or added relevant lines in 7 files are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.1%) to 89.848%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/transpiler/passes/routing/sabre_swap.py	6	7	85.71%
crates/accelerate/src/sabre/route.rs	70	74	94.59%
crates/accelerate/src/sabre/heuristic.rs	43	143	30.07%

Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	4	92.62%

Totals
Change from base Build 10164847368:	-0.1%
Covered Lines:	66404
Relevant Lines:	73907

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💛 - Coveralls