

TITLE
Problem converting slow tokenizer to fast: token out of vocabulary

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BODY
When I try to use a [Dutch RoBERTa model](https://huggingface.co/pdelobelle/robbert-v2-dutch-base/tree/main#how-to-use) as suggested, the library tries to convert the old (slow) tokenizer to the fast one. However, this leads to issues. (I can just keep the slow one, but I need to use the offset and word_ids functionality which is only available in the fast tokenizers.)

```python
from transformers import AutoTokenizer

tokenizer = AutoTokenizer.from_pretrained("pdelobelle/robbert-v2-dutch-base")
```

Error trace:

```
 File "C:/dev/python/jasper-tok2vec/main.py", line 10, in main
    tokenizer = AutoTokenizer.from_pretrained("pdelobelle/robbert-v2-dutch-base")
  File "C:\Users\bramv\.virtualenvs\jasper-tok2vec-4kc9ajCV\lib\site-packages\transformers\models\auto\tokenization_auto.py", line 378, in from_pretrained
    return tokenizer_class_fast.from_pretrained(pretrained_model_name_or_path, *inputs, **kwargs)
  File "C:\Users\bramv\.virtualenvs\jasper-tok2vec-4kc9ajCV\lib\site-packages\transformers\tokenization_utils_base.py", line 1804, in from_pretrained
    return cls._from_pretrained(
  File "C:\Users\bramv\.virtualenvs\jasper-tok2vec-4kc9ajCV\lib\site-packages\transformers\tokenization_utils_base.py", line 1877, in _from_pretrained
    tokenizer = cls(*init_inputs, **init_kwargs)
  File "C:\Users\bramv\.virtualenvs\jasper-tok2vec-4kc9ajCV\lib\site-packages\transformers\models\roberta\tokenization_roberta_fast.py", line 160, in __init__
    super().__init__(
  File "C:\Users\bramv\.virtualenvs\jasper-tok2vec-4kc9ajCV\lib\site-packages\transformers\models\gpt2\tokenization_gpt2_fast.py", line 133, in __init__
    super().__init__(
  File "C:\Users\bramv\.virtualenvs\jasper-tok2vec-4kc9ajCV\lib\site-packages\transformers\tokenization_utils_fast.py", line 89, in __init__
    fast_tokenizer = convert_slow_tokenizer(slow_tokenizer)
  File "C:\Users\bramv\.virtualenvs\jasper-tok2vec-4kc9ajCV\lib\site-packages\transformers\convert_slow_tokenizer.py", line 642, in convert_slow_tokenizer
    return converter_class(transformer_tokenizer).converted()
  File "C:\Users\bramv\.virtualenvs\jasper-tok2vec-4kc9ajCV\lib\site-packages\transformers\convert_slow_tokenizer.py", line 262, in converted
    BPE(
Exception: Error while initializing BPE: Token `Ċ` out of vocabulary
```

### Environment

- `transformers` version: 4.1.1
- Platform: Windows-10-10.0.19041-SP0
- Python version: 3.8.2
- PyTorch version (GPU?): 1.7.1+cu110 (True)
- Tensorflow version (GPU?): not installed (NA)


### Who can help
 
@mfuntowicz