Fix error when training multilingual_translation task with multi-GPU

Summary:
D10052908 introduce multilingual_translation task, but it raises exception when training with multiple-GPUs: P60202593

With Myle's help, we found that it is because of improperly handled dummy batch data type, and it causes optimizer.backward() is not executed same number of times cross different GPUs.

Reviewed By: xianxl

Differential Revision: D12964263

fbshipit-source-id: 4991039030bf373f0c484e131acc4736487be4d8

fairseq/data/round_robin_zip_datasets.py

@@ -59,6 +59,8 @@ def __len__(self):
 
     def collater(self, samples):
         """Merge a list of samples to form a mini-batch."""
+        if len(samples) == 0:
+            return None
         if self.eval_key is None:
             return OrderedDict([
                 (key, dataset.collater([sample[key] for sample in samples]))