Prevent negative edge weights in Graclus

Change to flake8 standards and move to first weight is not None call

Author: vertumnal

torch_cluster/graclus.py

@@ -39,6 +39,9 @@ def graclus_cluster(
     row, col = row[mask], col[mask]
 
     if weight is not None:
+        if (weight < 0).any().item():
+            raise ValueError(f"PyG's implementation of Graclus clustering "
+                             f"does not support negative edge weights.")
         weight = weight[mask]
 
     # Randomly shuffle nodes.
@@ -51,9 +54,6 @@ def graclus_cluster(
     row, col = row[perm], col[perm]
 
     if weight is not None:
-        if (weight < 0).any().item():
-            raise ValueError(f"PyG's implementation of Graclus clustering "
-                            f"does not support negative edge weights.")
         weight = weight[perm]
 
     deg = row.new_zeros(num_nodes)