Support wire cutting of QubitCircuit

[sansiro77]

cir = dq.QubitCircuit(7)
for i in range(7):
    cir.rx(i, np.pi / 4)
cir.cut(6)
cir.cx(0, 3)
cir.cx(1, 3)
cir.cx(2, 3)
cir.cut(3)
cir.cx(3, 4)
cir.cx(3, 5)
cir.cx(3, 6)
cir.cut(3)
cir.cx(0, 3)
cir.cx(1, 3)
cir.cx(2, 3)
cir.observable(6)
cir.observable(3)
cir.observable(0)

subexperiments, coefficients = cir.get_subexperiments()

shots = 1024
expval_total = 0
for i, coeff in enumerate(coefficients):
    expval = torch.ones(3) * coeff
    for label, subcircuits in subexperiments.items():
        subcircuits[i]()
        expval_per_label = subcircuits[i].expectation(shots=shots)
        expval *= expval_per_label
    expval_total += expval
reconstructed_expval = expval_total.sum()

[xiangmind]

LGTM.