[gen utils] missing else case
This code: https://github.com/huggingface/transformers/blob/master/src/transformers/generation_utils.py#L361-L369
```
        # current position and vocab size
        if hasattr(self.config, "vocab_size"):
            vocab_size = self.config.vocab_size
        elif (
            self.config.is_encoder_decoder
            and hasattr(self.config, "decoder")
            and hasattr(self.config.decoder, "vocab_size")
        ):
            vocab_size = self.config.decoder.vocab_size
```
1. `else` is missing - I hit that case while porting a model. Probably needs to assert there?
```
raise ValueError("either self.config.vocab_size or self.config.decoder.vocab_size need to be defined")
```

2. also the comment on top seems to be outdated (just `vocab_size` is being set there)

sent PR https://github.com/huggingface/transformers/pull/6980