apply method not working properly for gates.GeneralizedRBS

[renatomello]

As per title. Here is a minimal code to reproduce the bug.

As seen in #1591 , the problem seems to be either in

qibo/src/qibo/backends/einsum_utils.py

Line 28 in 725e646

def apply_gate_string(qubits, nqubits):

or

qibo/src/qibo/backends/einsum_utils.py

Line 10 in 725e646

def prepare_strings(qubits, nqubits):

In [1]: import numpy as np
   ...: 
   ...: from qibo import gates
   ...: from qibo.backends import NumpyBackend
   ...: 
   ...: backend = NumpyBackend()
   ...: 
   ...: gate = gates.GeneralizedRBS([1], [0, 2], 0.1)
   ...: matrix = gate.matrix(backend)
   ...: 
   ...: state = np.array([0, 0, 1, 0, 0, 0, 0, 0], dtype=complex)
   ...: 
   ...: print(matrix @ state)
   ...: print()
   ...: print(gate.apply(backend, state, nqubits=3))
[0.        +0.j 0.        +0.j 0.99500417+0.j 0.        +0.j
 0.        +0.j 0.09983342+0.j 0.        +0.j 0.        +0.j]

[0.+0.j 0.+0.j 1.+0.j 0.+0.j 0.+0.j 0.+0.j 0.+0.j 0.+0.j]