Change output to QuantumState class for quimb backend

src/qibotn/backends/quimb.py

@@ -1,6 +1,6 @@
 from qibo.backends.numpy import NumpyBackend
 from qibo.config import raise_error
-from qibo.result import CircuitResult
+from qibo.result import QuantumState
 
 
 class QuimbBackend(NumpyBackend):
@@ -80,5 +80,4 @@ class QuimbBackend(NumpyBackend):
         if return_array:
             return state.flatten()
         else:
-            circuit._final_state = CircuitResult(self, circuit, state.flatten(), nshots)
-            return circuit._final_state
+            return QuantumState(state.flatten())