Metric docs fix

pytorch_lightning/metrics/functional/classification.py

@@ -82,7 +82,8 @@ def get_num_classes(
             num_classes = pred.size(1)
         else:
             num_target_classes = int(target.max().detach().item() + 1)
-            num_classes = num_target_classes
+            num_pred_classes = int(pred.max().detach().item() + 1)
+            num_classes = max(num_target_classes, num_pred_classes)
     return num_classes