Fix backtranslation dataset on IndexedCachedDataset (#410)

Summary:
BacktranslationDataset would throw an error when the underlying dataset was an IndexedCachedDataset because prefetching was not handled correctly. This fixes the error.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/410

Differential Revision: D13557539

Pulled By: myleott

fbshipit-source-id: 398ab59a3ebdbf1c666d862b9f905654eece800c

fairseq/data/backtranslation_dataset.py

@@ -166,3 +166,11 @@ class BacktranslationDataset(FairseqDataset):
         """
         tgt_size = self.tgt_dataset.size(index)[0]
         return (tgt_size, tgt_size)
+    
+    @property
+    def supports_prefetch(self):
+        return self.tgt_dataset.supports_prefetch()
+
+    def prefetch(self, indices):
+        return self.tgt_dataset.prefetch(indices)
+