Fix IoUs reported in segmentation references (#7916)

Co-authored-by: Nicolas Hug <nh.nicolas.hug@gmail.com>

references/segmentation/utils.py

@@ -88,7 +88,7 @@ class ConfusionMatrix:
         return acc_global, acc, iu
 
     def reduce_from_all_processes(self):
-        reduce_across_processes(self.mat)
+        self.mat = reduce_across_processes(self.mat).to(torch.int64)
 
     def __str__(self):
         acc_global, acc, iu = self.compute()