'DistilBertModel' object has no attribute 'init_weights'
Author: xeb
Pull request: no

## 🐛 Bug

<!-- Important information -->

Model I am using (Bert, XLNet....): DistilBertModel

Language I am using the model on (English, Chinese....): English

The problem arise when using:
* [x] the official example scripts: I am trying to run the sample in the given examples within a notebook. Specifically the example in [DistilBERT's Example README](https://github.com/huggingface/pytorch-transformers/tree/master/examples/distillation)


## To Reproduce

Steps to reproduce the behavior:

1. Create a new notebook with a Python 3.7 interpretter
2. Type in the following code:
```
!pip install pytorch-transformers
import torch
from pytorch_transformers.tokenization_distilbert import DistilBertTokenizer
from pytorch_transformers.modeling_distilbert import DistilBertModel
tokenizer = DistilBertTokenizer.from_pretrained('distilbert-base-uncased')
model = DistilBertModel.from_pretrained('distilbert-base-uncased')

input_ids = torch.tensor(tokenizer.encode("Hello, my dog is cute")).unsqueeze(0)
outputs = model(input_ids)
last_hidden_states = outputs[0]  # The last hidden-state is the first element of the output tuple
```
3. Run & receive the error:
```
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-48-3aa5cae06e9c> in <module>
      1 tokenizer = DistilBertTokenizer.from_pretrained('distilbert-base-uncased')
----> 2 model = DistilBertModel.from_pretrained('distilbert-base-uncased')
      3 
      4 input_ids = torch.tensor(tokenizer.encode("Hello, my dog is cute")).unsqueeze(0)
      5 outputs = model(input_ids)

/mnt/c/Users/.../venv/lib/python3.7/site-packages/pytorch_transformers/modeling_utils.py in from_pretrained(cls, pretrained_model_name_or_path, *model_args, **kwargs)
    472             assert model.config.output_attention == True
    473             # Loading from a TF checkpoint file instead of a PyTorch model (slower)
--> 474             config = BertConfig.from_json_file('./tf_model/my_tf_model_config.json')
    475             model = BertModel.from_pretrained('./tf_model/my_tf_checkpoint.ckpt.index', from_tf=True, config=config)
    476 

/mnt/c/Users/.../venv/lib/python3.7/site-packages/pytorch_transformers/modeling_distilbert.py in __init__(self, config)
    486         self.transformer = Transformer(config) # Encoder
    487 
--> 488         self.init_weights()
    489 
    490     def _resize_token_embeddings(self, new_num_tokens):

/mnt/c/Users/.../venv/lib/python3.7/site-packages/torch/nn/modules/module.py in __getattr__(self, name)
    589                 return modules[name]
    590         raise AttributeError("'{}' object has no attribute '{}'".format(
--> 591             type(self).__name__, name))
    592 
    593     def __setattr__(self, name, value):

AttributeError: 'DistilBertModel' object has no attribute 'init_weights'
```

## Expected behavior
Code would execute. Not sure if init_weights should be inherited or if it's a type (there is [_init_weights ](https://github.com/huggingface/pytorch-transformers/blob/master/pytorch_transformers/modeling_distilbert.py#L403)in the DistilBertTokenizer class)

## Environment

* OS: Win10 (with WSL)
* Python version: 3.7.4
* PyTorch version: 1.2
* PyTorch Transformers version (or branch): mainline
* Using GPU ? no
* Distributed of parallel setup ? parallel


Comments:
Hello, and thank you for the bug report! It would seem you are using an outdated version of the master branch. Could you update it to the latest and tell me if the error remains?

Looks good! I pulled the latest pip package and that worked as well. Thanks.