update: add from_state_dict for PreTrainedBertModel
For restoring the training procedure. 
Now we can use torch.save to store their model and restore their model by e.g. `model = BertForSequenceClassification.from_state_dict('bert-large-uncased', state_dict=torch.load('xx.pth'))`
Hi, I like the idea but I not a big fan of all the code duplication I'ld rather fuse the two loading functions in one.
Basically we can just add a `state_dict` argument to `from_pretrained` and add a check in `from_pretrained` to handle the case.<|||||>> Hi, I like the idea but I not a big fan of all the code duplication I'ld rather fuse the two loading functions in one.
> Basically we can just add a `state_dict` argument to `from_pretrained` and add a check in `from_pretrained` to handle the case.

Hi, thanks for your reply, I just tried to make sure that my code will not incorporate bugs so I added a new function instead of changing code in `from_pretrained`.
:)