Update data_utils.py for (#598)

Summary:
Correcting the syntax error in assert function cause of a character before error message.

Assertion and the code is working fine now, Tested with wmt-ende task.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/598

Differential Revision: D14712846

Pulled By: myleott

fbshipit-source-id: 3f708aa2362ceecba19174750f9ffc9238537512

fairseq/data/data_utils.py

@@ -156,7 +156,7 @@ def batch_by_size(
     for idx in indices:
         sample_lens.append(num_tokens_fn(idx))
         sample_len = max(sample_len, sample_lens[-1])
-        assert sample_len <= max_tokens, f"sentence at index {idx} exceeds max_tokens limit!"
+        assert sample_len <= max_tokens, "sentence at index {idx} exceeds max_tokens limit!".format(idx=idx)
         num_tokens = (len(batch) + 1) * sample_len
         if is_batch_full(num_tokens):
             mod_len = max(