Update seq2seq_utils.py

mistake in BLEU Score

Author: Hamel Husain

notebooks/seq2seq_utils.py

@@ -421,5 +421,9 @@ def evaluate_model(self, holdout_bodies, holdout_titles):
             predicted.append(self.pp_title.process_text([yhat])[0])
         # calculate BLEU score
         logging.warning('Calculating BLEU.')
-        bleu = corpus_bleu(actual, predicted)
+        
+        #must be careful with nltk api for corpus_bleu!, 
+        # expects List[List[List[str]]] for ground truth, if you forget you will get
+        # errenous results!
+        bleu = corpus_bleu([[a] for a in actual], predicted)
         return bleu