Bug in qibo.transpiler.placer.Subgraph for Qibo 0.2.14 or master

[carlos-luque]

The if statement that checks the number of two-qubit gates in the circuit contains an error. The statement checks if there are three two-qubit gates.
The if statement on line 129 is located in the function call of the class Subgraph. https://github.com/qiboteam/qibo/blob/45886717471a919d09a832a953c1c05cb6174fa1/src/qibo/transpiler/placer.py#L129C1-L134C14

To Reproduce

import networkx as nx

def star_connectivity():
    Q = [i for i in range(5)]
    chip = nx.Graph()
    chip.add_nodes_from(Q)
    graph_list = [(Q[i], Q[2]) for i in range(5) if i != 2]
    chip.add_edges_from(graph_list)
    return chip

QPU = star_connectivity()

import qibo
from qibo import gates
from qibo.models import Circuit

qibo.set_backend("numpy")

circuit = Circuit(4)
circuit.add(gates.H(0))
circuit.add(gates.CNOT(0, 1))
circuit.add(gates.CNOT(1, 2))
circuit.add(gates.M(3))

from qibo.transpiler.optimizer import Preprocessing
from qibo.transpiler.placer import Subgraph

fase_asignacion = Preprocessing(connectivity=QPU)
asignacion_circuit = fase_asignacion(circuit)
placement = Subgraph(connectivity=QPU)
colocacion_incial = placement(asignacion_circuit)

Error:
[Qibo 0.2.14|ERROR|.....]: Circuit must contain at least two two-qubit gates to implement subgraph placement.

Expected behavior
The circuit contains at least two two-qubit gates, so the error shouldn't occur.

Sugested fix
I would suggest changing the number 3 to 2 in the if statement that checks the number of two-qubit gates in the circuit

[alecandido]

Thanks @carlos-luque for reporting the issue.

As you said, it is pretty much an easy fix. But since you found it, I would encourage you to contribute a PR with the proposed suggestion, and a corresponding test (which should ensure that the fix is working, but also that the problem will not appear again).

[carlos-luque]

Thanks @alecandido for you reply. I'll do it