Right now sabre layout uses n random trials (as specified by the user or
defaulting to num_cpus) and since Qiskit#12453 one additional trial taking the
qubits of the densest subgraph as a starting point. There are also a
couple of other trivial examples to try which may or may not produce
better results depending on the circuit, a trivial layout and a reverse
trivial layout. In the case of a hardware efficient circuit the trivial
layout will map exactly and would always be picked. When running in a
preset pass manager sabre should never be called in this scenario
because VF2Layout will find the mapping, but the incremental cost of
adding the trial is minimal. Similarly the cost of a reversed trivial
layout (where virtual qubit 0 -> n, 1 -> n - 1, etc.) is trivial and
may in some scenarios produce a better results.

…ials

Co-authored-by: Elena Peña Tapia <[email protected]>