Improved Documentation and Error-Signaling of VF2PostLayout

[sbrandhsn]

Summary

Fixes #11089 as discussed with @mtreinish

Details and comments

VF2PostLayout does not return a solution when presented with an already ideal initial layout for a quantum circuit. This PR extends the VF2PostLayout pass by a 'no better solution found' error code for these situations and changes the documentation accordingly.

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[jakelishman]

Thanks for looking at this! Please could you also add a release note with an "upgrade" note that explains that VF2PostLayout now distinguishes the cases of "nothing better" and "no solution"?

[jakelishman]

Sorry for the really long delay in coming back to this (and my misreading of what you'd done in the prior review!). The code looks good to merge to me, I just had a minor comment about the release note.

[jakelishman]

Minor: VF2PostLayout and Layout would best as Sphinx cross-references, so users can click them and jump to the full documentation on them. The syntax looks like this:

Suggested change

	VF2PostLayout no distinguishes between 'no solution' and 'no better solution' when determining a
	Layout for a given quantum circuit. 'no better solution' is set when the initial layout of a quantum
	circuit is also the optimal one, i.e. incurs the least cost in terms of error rates.
	:class:`.VF2PostLayout` no distinguishes between 'no solution' and 'no better solution' when determining a
	:class:`.Layout` for a given quantum circuit. 'no better solution' is set when the initial layout of a quantum
	circuit is also the optimal one, i.e. incurs the least cost in terms of error rates.

[brandhsn]

Sure, I changed it, thanks for the feedback on the release note! :)

[brandhsn]

No worries! Thanks for your input, I changed the release notes!

[coveralls]

Pull Request Test Coverage Report for Build 6743723348

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A quick fix for this PR: rebase it; your next report should be accurate.

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• 1980 unchanged lines in 132 files lost coverage.
• Overall coverage increased (+0.002%) to 86.9%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/algorithms/evolvers/real_evolver.py	1	92.31%
qiskit/algorithms/minimum_eigen_solvers/minimum_eigen_solver.py	1	97.78%
qiskit/algorithms/phase_estimators/phase_estimation_result.py	1	96.36%
qiskit/circuit/classicalregister.py	1	90.48%
qiskit/circuit/quantumregister.py	1	93.33%
qiskit/opflow/converters/converter_base.py	1	91.67%
qiskit/opflow/evolutions/evolution_base.py	1	92.31%
qiskit/opflow/expectations/matrix_expectation.py	1	96.15%
qiskit/opflow/gradients/circuit_qfis/circuit_qfi.py	1	93.33%
qiskit/opflow/gradients/gradient_base.py	1	96.15%

Totals
Change from base Build 6612358367:	0.002%
Covered Lines:	74044
Relevant Lines:	85206

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💛 - Coveralls

[jakelishman]

Thanks for the changes!