fix the bugs of mixup

fairseq/models/speech_to_text/s2t_transformer.py

@@ -899,6 +899,7 @@ class S2TTransformerEncoder(FairseqEncoder):
 
         layer_idx = -1
         mixup = None
+        if self.mixup:
             if type(self.mixup_layer) is list:
                 mixup_layer = choice(self.mixup_layer)
             else: