Init BertForTokenClassification from from_pretrained
```
model = BertForTokenClassification.from_pretrained('bert-base-uncased', 2)
```
will complain about missing positional arg for `num_labels`. 
The root cause is here the function signature should actually be 
https://github.com/huggingface/pytorch-pretrained-BERT/blob/19666dcb3bee3e379f1458e295869957aac8590c/pytorch_pretrained_bert/modeling.py#L522
```
    def from_pretrained(cls, pretrained_model_name_or_path,  *inputs, state_dict=None, cache_dir=None, from_tf=False, **kwargs):
``` 
But note that the signature above above is actually only supported in py3 not py2. See a similar workaround here: https://github.com/pytorch/pytorch/pull/19247/files#diff-bdb85c31edc2daaad6cdb68c0d19bafbR300
actually this is related to my current work, I will send a fix along with my PR.