feat: changed backend generation mechanism + updated tutorial

src/qibotn/backends/__init__.py

@@ -4,7 +4,6 @@ from qibo.config import raise_error
 
 from qibotn.backends.abstract import QibotnBackend
 from qibotn.backends.cutensornet import CuTensorNet  # pylint: disable=E0401
-from qibotn.backends.quimb import QuimbBackend
 
 PLATFORMS = ("cutensornet", "qutensornet", "qmatchatea")
 
@@ -26,9 +25,11 @@ class MetaBackend:
         if platform == "cutensornet":  # pragma: no cover
             return CuTensorNet(runcard)
         elif platform == "quimb":  # pragma: no cover
+            import qibotn.backends.quimb as qmb
+
             quimb_backend = kwargs.get("quimb_backend", "numpy")
             contraction_optimizer = kwargs.get("contraction_optimizer", "auto-hq")
-            return QuimbBackend(
+            return qmb.BACKENDS[quimb_backend](
                 quimb_backend=quimb_backend, contraction_optimizer=contraction_optimizer
             )
         elif platform == "qmatchatea":  # pragma: no cover