Track swap mapping permutations in Stochastic mapper

[nonhermitian]

Summary

Being able to determine the final qubit permutation after swap mapping has been a much needed routine for a couple of years now. It is trivial to implement and I have done so here for the Stochastic swap mapper.

Left to do before removing WIP:

• Add tests.
• Changelog

Details and comments

[nonhermitian]

Back into WIP as there is edge cases

[itoko]

All the routing passes (including StochasticSwap) now seem to set the final permutation as a Layout object to property_set["final_layout"] when fake_run option is True.
Why don't we promote property_set["final_layout"] to be always set in run method (for all routing passes), instead of introducing a new property final_permutation?

[nonhermitian]

So there are a couple of advantages here: 1) This computes without the need for a fake run. 2) This includes the initial layout in the permutation so that this tells you the full permutation details whereas the method you bring up does not. Importantly, this will allow you to put measurements and single qubit gates on a circuit AFTER compilation in the correct order.