transpiled circuit layout refers to original circuit

[1ucian0]

A recent issue raised the problem of how confusing a transpiled circuit layout is. In other words, the result of transpiling circuit gives you a result_circuit in which result_circuit._layout is in the domain of circuit.

Should the transpiled circuit make the change to the flat register q or be kept in the original naming (holding the layout)?

[jakelishman]

The Layout linking the physical qubits of result_circuit to the virtual ones of circuit was always necessary, though it's unfortunate that Layout makes the physical qubits harder to work with in the output.

That said, the spirit of the problem should have been solved by #10835, so the TranspileLayout class that's now the public QuantumCircuit.layout field includes methods to get things like the initial layout suitable for recompilation.