bug: fixed StackedDistributedArray.norm to work with cupy arrays

pylops_mpi/DistributedArray.py

@@ -818,21 +818,22 @@ def norm(self, ord: Optional[int] = None):
         ord : :obj:`int`, optional
             Order of the norm.
         """
-        norms = np.array([distarray.norm(ord) for distarray in self.distarrays])
+        ncp = get_module(self.distarrays[0].engine)
+        norms = ncp.array([distarray.norm(ord) for distarray in self.distarrays])
         ord = 2 if ord is None else ord
         if ord in ['fro', 'nuc']:
             raise ValueError(f"norm-{ord} not possible for vectors")
         elif ord == 0:
             # Count non-zero then sum reduction
-            norm = np.sum(norms)
-        elif ord == np.inf:
+            norm = ncp.sum(norms)
+        elif ord == ncp.inf:
             # Calculate max followed by max reduction
-            norm = np.max(norms)
-        elif ord == -np.inf:
+            norm = ncp.max(norms)
+        elif ord == -ncp.inf:
             # Calculate min followed by max reduction
-            norm = np.min(norms)
+            norm = ncp.min(norms)
         else:
-            norm = np.power(np.sum(np.power(norms, ord)), 1. / ord)
+            norm = ncp.power(ncp.sum(ncp.power(norms, ord)), 1. / ord)
         return norm
 
     def conj(self):