[gen utils] missing else case (#6980)

* [gen utils] missing else case

1. `else` is missing - I hit that case while porting a model. Probably needs to assert there?
2. also the comment on top seems to be outdated (just vocab_size is being set there)

* typo

src/transformers/generation_utils.py

@@ -358,7 +358,7 @@ class GenerationMixin:
             )
             pad_token_id = eos_token_id
 
-        # current position and vocab size
+        # vocab size
         if hasattr(self.config, "vocab_size"):
             vocab_size = self.config.vocab_size
         elif (
@@ -367,6 +367,8 @@ class GenerationMixin:
             and hasattr(self.config.decoder, "vocab_size")
         ):
             vocab_size = self.config.decoder.vocab_size
+        else:
+            raise ValueError("either self.config.vocab_size or self.config.decoder.vocab_size needs to be defined")
 
         # set effective batch size and effective batch multiplier according to do_sample
         if do_sample: