CupyBackend matrices are np.ndarrays

[BrunoLiegiBastonLiegi]

While working on qiboteam/qibo#1548 I noticed that if you try to get the matrices from the CupyBackend, namely:

from qibojit.backends import CupyBackend

b = CupyBackend()
print(type(b.matrices.X))

you get a np.ndarray rather than a cp.ndarray. This does not seem to be the case for other backends that do not have np.ndarray as native arrays, e.g. PyTorchBackend or Tensorflowbackend.
I was wondering what was the reason behind this, because this makes some things in qiboteam/qibo#1548 more annoying, requiring for you to explicitely cast to cupy arrays when needed.