Fix stories generation

Summary: Pull Request resolved: https://github.com/pytorch/fairseq/pull/454

Differential Revision: D13708565

Pulled By: myleott

fbshipit-source-id: 5cd0e07e3e1885eef14e3a5e8074f24cf4bde632

fairseq/models/fconv_self_att.py

@@ -245,9 +245,9 @@ class FConvEncoder(FairseqEncoder):
             eo.index_select(0, new_order) for eo in encoder_out['encoder_out']
         )
 
-        encoder_out['encoder_padding_mask'] = tuple(
-            eo.index_select(0, new_order) for eo in encoder_out['encoder_padding_mask']
-        )
+        if encoder_out['encoder_padding_mask'] is not None:
+            encoder_out['encoder_padding_mask'] = \
+                encoder_out['encoder_padding_mask'].index_select(0, new_order)
 
         if 'pretrained' in encoder_out:
             encoder_out['pretrained']['encoder_out'] = tuple(