Pretokenized text handling mistake in  tokenization_utils.py file
Hi, 

I'm wondering if this is a mistake that in the `tokenization_utils.py` file, the pretokenized text will be further tokenized, while the raw text would not. The code is as follows: 

```
if is_pretokenized:
    tokens = list(itertools.chain(*(self.tokenize(t, is_pretokenized=True, **kwargs) for t in text)))
         return self.convert_tokens_to_ids(tokens)
    else:
        return self.convert_tokens_to_ids(text)
```

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