Fix operator norm device (#520)

Fixes #512

src/mrpro/operators/LinearOperator.py

@@ -163,7 +163,7 @@ def operator_norm(
         # operator norm is a strictly positive number. This ensures that the first time the
         # change between the old and the new estimate of the operator norm is non-zero and
         # thus prevents the loop from exiting despite a non-correct estimate.
-        op_norm_old = torch.zeros(*tuple([1 for _ in range(vector.ndim)]))
+        op_norm_old = torch.zeros(*tuple([1 for _ in range(vector.ndim)]), device=vector.device)
 
         dim = tuple(dim) if dim is not None else dim
         for _ in range(max_iterations):