max_len returns unexpected value

Hi,

I noticed something weird about the `max_len` attribute of the `tokenizer`

```python
from transformers import AutoTokenizer

tokenizer = AutoTokenizer.from_pretrained("allenai/scibert_scivocab_uncased")
print(tokenizer.max_len) # => 1000000000000000019884624838656
```

Whereas I expected it to be 512, as in

```python
tokenizer = AutoTokenizer.from_pretrained("bert-base-uncased")
print(tokenizer.max_len) # => 512
```

is this a bug? Or is `max_len` not the appropriate attribute to use if I want to know the max length for the inputs of the model?