fix iterator when loading from checkpoint (#5344)

Co-authored-by: Junteng Jia <juntengjia@fb.com>

fairseq/data/iterators.py

@@ -524,7 +524,7 @@ def _get_iterator_for_epoch(
             # TODO: Below is a lazy implementation which discard the final batch regardless
             # of whether it is a full batch or not.
 
-            total_num_itrs = len(self.epoch_batch_sampler) - 1
+            total_num_itrs = len(itr) - 1
             itr.take(total_num_itrs)
             logger.info(f"skip final residual batch, total_num_itrs = {total_num_itrs}")