fix string format to work in python 3.5 (#1050)

Summary:
change string fromat in fairseq/data/subsample_dataset.py#20
Pull Request resolved: https://github.com/pytorch/fairseq/pull/1050

Differential Revision: D16946060

Pulled By: okhonko

fbshipit-source-id: 0eabf22e7ffd4f658b6d18c87dc6e59c81a355c7

fairseq/data/subsample_dataset.py

@@ -17,7 +17,7 @@ class SubsampleDataset(BaseWrapperDataset):
             range(len(self.dataset)), self.actual_size, replace=False
         )
         print(
-            f"subsampled dataset from {len(self.dataset)} to {self.actual_size} (ratio={size_ratio})"
+            "subsampled dataset from {} to {} (ratio={})".format(len(self.dataset), self.actual_size, size_ratio)
         )
 
     def __getitem__(self, index):