fix the bugs of mixup

fairseq/models/speech_to_text/s2t_transformer.py

@@ -899,10 +899,11 @@ class S2TTransformerEncoder(FairseqEncoder):
 
         layer_idx = -1
         mixup = None
-        if type(self.mixup_layer) is list:
-            mixup_layer = choice(self.mixup_layer)
-        else:
-            mixup_layer = self.mixup_layer
+        if self.mixup:
+            if type(self.mixup_layer) is list:
+                mixup_layer = choice(self.mixup_layer)
+            else:
+                mixup_layer = self.mixup_layer
 
         if self.history is not None:
             self.history.clean()