Setting qibolab backend

[andrea-pasquale]

I'm having a hard time try to set qibolab as a backend in the latest release 0.2.5.

from qibo import set_backend

set_backend("qibolab", platform="dummy")

The following error is raised:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/andreapasquale/test_env/lib/python3.11/site-packages/qibo/backends/__init__.py", line 152, in set_backend
    GlobalBackend.set_backend(backend, **kwargs)
  File "/home/andreapasquale/test_env/lib/python3.11/site-packages/qibo/backends/__init__.py", line 112, in set_backend
    log.info(f"Using {cls._instance} backend on {cls._instance.device}")
                                                 ^^^^^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'device'