move function from quantum_info.random_ensembles to `backends.__ini…

…t__`

src/qibo/backends/__init__.py

@@ -1,6 +1,7 @@
 import os
 
-from qibo.backends.abstract import Backend
+import numpy as np
+
 from qibo.backends.clifford import CliffordBackend
 from qibo.backends.npmatrices import NumpyMatrices
 from qibo.backends.numpy import NumpyBackend
@@ -200,3 +201,28 @@ def _check_backend(backend):
         return GlobalBackend()
 
     return backend
+
+def _check_backend_and_local_state(seed, backend):
+    if (
+        seed is not None
+        and not isinstance(seed, int)
+        and not isinstance(seed, np.random.Generator)
+    ):
+        raise_error(
+            TypeError, "seed must be either type int or numpy.random.Generator."
+        )
+
+    backend = _check_backend(backend)
+
+    if seed is None or isinstance(seed, int):
+        if backend.__class__.__name__ in [
+            "CupyBackend",
+            "CuQuantumBackend",
+        ]:  # pragma: no cover
+            local_state = backend.np.random.default_rng(seed)
+        else:
+            local_state = np.random.default_rng(seed)
+    else:
+        local_state = seed
+
+    return backend, local_state