Qiskit / qiskit / 17692918299

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Commit Message

Use on-the-fly scoring in VF2Layout (#14857)

* Use on-the-fly scoring in VF2Layout

This uses the new on-the-fly scoring capabilities of our VF2
implementation in the Rust-native version of the VF2Layout pass, as the
scoring system was designed to be used.  In particular: the
`score_internal` function is no longer necessary at all, because the
scoring is now done entirely on-the-fly during the isomorphism iterator.

The structure of the interaction and coupling graphs is changed by this
patch to support the scoring:

- The virtual-interaction graph is now always directed, and the node and
  edge weights store the number of operations on the corresponding
  `qargs` sequence.

- The physical-coupling graph is now also always directed, and the node
  and edge weights are the negative logarithms of the "fidelities"
  implied by the arithmetically averaged error rates.  This metric is
  perhaps not ideal from a mathematical perspective (if nothing else,
  the geometric mean of the errors might make more sense), but the
  averaging matches the previous implementation in scoring.  The graph
  weights are the negatives of the log-fidelities, so that the score is
  non-decreasing (since the VF2 implementation frames the reduction
  as a minimisation problem) and so that the combination of two scores
  is done by addition and multiplication rather than multiplication and
  (integer) exponentiation, which should be cheaper.

- `strict_direction=False` is now handled by cloning and reversing any
  edges of the physical-coupling graph that have no reversed
  counterpart, rather than by changing the graph structure itself.

- All graphs constructed for the isomorphism checker are now base
  `Graph` instances, rather than `StableGraph`.  This saves space in the
  backing data structures, and should help cache locality in them.  This
  is achieved by reworking the construction logic of the
  virtual-interaction graph so that never needs to remove nodes from the
  gr... (continued)

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39 existing lines in 7 files now uncovered.

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