[Fix] too many open files (#3694)

* [Fix] too many open files

python/dgl/distributed/dist_dataloader.py

@@ -155,5 +155,11 @@ class DistDataLoader:
             end_pos = self.current_pos + self.batch_size
         idx = self.data_idx[self.current_pos:end_pos].tolist()
         ret = [self.dataset[i] for i in idx]
+        # Sharing large number of tensors between processes will consume too many
+        # file descriptors, so let's convert each tensor to scalar value beforehand.
+        if isinstance(ret[0], tuple):
+            ret = [(type, F.as_scalar(id)) for (type, id) in ret]
+        else:
+            ret = [F.as_scalar(id) for id in ret]
         self.current_pos = end_pos
         return ret