CTRL tokenizer has no special tokens to indicate EOS
## 🐛 Bug

The `generate` method from `PreTrainedModel` by default uses index 0 as EOS. This is a problem with CTRL, because its tokenizer has the word `the` mapped to this id.

Actually the CTRL has no special tokens besides UNK:

```
tokenizer = CTRLTokenizer.from_pretrained('ctrl')
tokenizer.special_tokens_map                                                                                                                                                                             
# {'unk_token': '<unk>'}

tokenizer.convert_ids_to_tokens([0])                                                                                                                                                                     
# ['the']
```

I believe the CTRL tokenizer should have some special token to use as EOS or PAD, as the other models do.
