BertModel' object missing 'save_pretrained' attribute
I was attempting to download a pre-trained BERT model & save it to my cloud directory using Google Colab.

model.save_pretrained() seems to be missing completely for some reason. 

Link to Colab notebook: https://colab.research.google.com/drive/1ix_nNhsd89nLfTy6Nyh-Ak8PHn1SYm-0

Here's my code:

```
!pip install pytorch_pretrained_bert

import torch
from pytorch_pretrained_bert import BertTokenizer, BertModel 
import pandas as pd
import numpy as np


### Let's download a model and tokenizer
model = BertModel.from_pretrained('bert-base-uncased')
tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')

### Now let's save our model and tokenizer to a directory
model.save_pretrained('./models/')
tokenizer.save_pretrained('./models/')

```

Error:
```
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-18-1a3b2c8b8e82> in <module>()
     11 
     12 ### Now let's save our model and tokenizer to a directory
---> 13 model.save_pretrained('./models/')
     14 tokenizer.save_pretrained('./models/')
     15 

/usr/local/lib/python3.6/dist-packages/torch/nn/modules/module.py in __getattr__(self, name)
    574                 return modules[name]
    575         raise AttributeError("'{}' object has no attribute '{}'".format(
--> 576             type(self).__name__, name))
    577 
    578     def __setattr__(self, name, value):
```

AttributeError: 'BertModel' object has no attribute 'save_pretrained'

Hi, `from_pretrained` appeared in an older version of the library. `pytorch-pretrained-BERT` is a year old, is less robust and lacks certain functionalities (such as the one you mentioned) which are present in `transformers`.