drawer enhancement: show layout info if circuit.layout exists

[ajavadia]

With some recent changes, a circuit that gets transpiled onto a coupling_map now will contain information about which qubits of the original (virtual) circuit correspond to which qubits of the final (physical) circuit. For example:

from qiskit.circuit import QuantumRegister, QuantumCircuit
from qiskit.compiler import transpile
from qiskit.test.mock import FakeTokyo
tokyo = FakeTokyo()

v = QuantumRegister(3)
circuit = QuantumCircuit(v)
circuit.h(v[0])
circuit.cx(v[0], v[1])
circuit.cx(v[0], v[2])

# do a GHZ state preparation on qubits 3, 7, 11, using ancillas for swap
new_circuit = transpile(circuit, tokyo, initial_layout=[3, 7, 11])

This request is to show the layout information next to each qubit. This can be found via:

new_circuit.layout

Layout({
3: Qubit(QuantumRegister(3, 'v'), 0),
7: Qubit(QuantumRegister(3, 'v'), 1),
11: Qubit(QuantumRegister(3, 'v'), 2),
0: Qubit(QuantumRegister(17, 'ancilla'), 0),
1: Qubit(QuantumRegister(17, 'ancilla'), 1),
2: Qubit(QuantumRegister(17, 'ancilla'), 2),
4: Qubit(QuantumRegister(17, 'ancilla'), 3),
5: Qubit(QuantumRegister(17, 'ancilla'), 4),
6: Qubit(QuantumRegister(17, 'ancilla'), 5),
8: Qubit(QuantumRegister(17, 'ancilla'), 6),
9: Qubit(QuantumRegister(17, 'ancilla'), 7),
10: Qubit(QuantumRegister(17, 'ancilla'), 8),
12: Qubit(QuantumRegister(17, 'ancilla'), 9),
13: Qubit(QuantumRegister(17, 'ancilla'), 10),
14: Qubit(QuantumRegister(17, 'ancilla'), 11),
15: Qubit(QuantumRegister(17, 'ancilla'), 12),
16: Qubit(QuantumRegister(17, 'ancilla'), 13),
17: Qubit(QuantumRegister(17, 'ancilla'), 14),
18: Qubit(QuantumRegister(17, 'ancilla'), 15),
19: Qubit(QuantumRegister(17, 'ancilla'), 16)
})

So I envision something like:

q0 (ancilla0) ------------
q1 (ancilla1) ------------
q2 (ancilla2) ------------
q3 (v0)       ------------
q4 (ancilla3) ------------
q5 (ancilla4) ------------
q6 (ancilla5) ------------
q7 (v1)       ------------
.
.

Note: the fact that we have q0, q1, .. q19 is an artifact of not being able to create register-less circuits currently. Eventually this should just be shown as physical qubits 0, 1, ..., 19.

[nonhermitian]

The example Layout is the reverse mapping, but other than that this is good.