Upgrade TranspileLayout

[pedrorrivero]

Summary

Upgrades TranspileLayout class adding one more field (i.e. output_qubit_mapping), a property, and a few convenience methods.

Details and comments

Field:

• output_qubit_mapping is added in order to make TranspileLayout self contained; meaning that it will not need to know about the (transpiled) circuit that it as attached to in order to understand its final_layout field. This mimics the way input_qubit_mapping allows interpretation of initial_layout.

Property:

• num_qubits is added as a convenient way to assess the number of qubits involved in the layout. Notice that checking the size of a dictionary is an $\mathcal{O}(1)$ operation, which justifies making this a property instead of a regular method (no computation is performed, only retrieval).

Methods:

• get_initial_permutation allows retrieval of the permutation performed by initial_layout.
• get_final_permutation allows retrieval of the permutation performed by final_layout.
• get_complete_permutation allows retrieval of the composed permutation from initial_layout and final_layout (in order).
• get_complete_layout allows retrieval of the complete layout that results from composing initial_layout and final_layout.

#9486 should be updated accordingly

[qiskit-bot]

Thank you for opening a new pull request.

Before your PR can be merged it will first need to pass continuous integration tests and be reviewed. Sometimes the review process can be slow, so please be patient.

While you're waiting, please feel free to review other open PRs. While only a subset of people are authorized to approve pull requests for merging, everyone is encouraged to review open pull requests. Doing reviews helps reduce the burden on the core team and helps make the project's code better for everyone.

One or more of the the following people are requested to review this:

• @Qiskit/terra-core
• @ikkoham

[pedrorrivero]

Kindly share your feedback @mtreinish @jakelishman @jlapeyre.

Please double check that I updated the three TranspileLayout instances appropriately.

[pedrorrivero]

An example of why setting final_layout to None instead of the trivial layout adds complexity @jakelishman @jlapeyre.

I would personally advocate for enforcing it (i.e. final_layout: Layout instead of final_layout: Optional[Layout] = None). Note that the trivial layout is actually trivial to build from output_qubit_mapping so, alternatively, this could be parsed automatically through __post_init__ or even retrieved through a getter.

[jakelishman]

I think this was obsoleted by #10835, but feel free to re-open if there's more to discuss.