Add model_max_length property on fast tokenizers
Fast tokenizers doesn't have model_max_length property as their "slow" counterpart.

```python
>>> t.model_max_len
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'BertTokenizerFast' object has no attribute 'model_max_len'
```



Signed-off-by: Morgan Funtowicz <morgan@huggingface.co>
I think this is handled in the base tokenizer class, Morgan.<|||||>I think I've got an issue in my dev env, something fucked up... 