ModuleNotFoundError: No module named 'pytorch_transformers.modeling' using convert_pytorch_checkpoint_to_tf.py
## 🐛 Bug

<!-- Important information -->

Model I am using (Bert, XLNet....):
BERT
Language I am using the model on (English, Chinese....):
Chinese
The problem arise when using:
* [✔️ ] the official example scripts: (give details)
run convert_pytorch_checkpoint_to_tf.py to generate the tf check point

## To Reproduce

Steps to reproduce the behavior:
    
```
python3  /Users/xxx/py3ml/lib/python3.6/site-packages/pytorch_transformers/convert_pytorch_checkpoint_to_tf.py
    Traceback (most recent call last):
  File "/Users/xxx/py3ml/lib/python3.6/site-packages/pytorch_transformers/convert_pytorch_checkpoint_to_tf.py", line 23, in <module>
    from pytorch_transformers.modeling import BertModel
ModuleNotFoundError: No module named 'pytorch_transformers.modeling'
```

## Expected behavior
convert it
<!-- A clear and concise description of what you expected to happen. -->

## Environment

* OS: macOS 10.14
* Python version: 3.6
* PyTorch version: 1.2
* PyTorch Transformers version (or branch):1.2.0
* Using GPU ? no
* Distributed of parallel setup ? no
* Any other relevant information:
I change the code to 
`from pytorch_transformer import BertModel`
It works fine


## Additional context

<!-- Add any other context about the problem here. -->
This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
