Torchscipt mode for BertForPreTraining
Hello, i used code from this tutorial https://huggingface.co/pytorch-transformers/torchscript.html
pytorch-transformers==1.0.0

```
from pytorch_pretrained_bert import BertModel, BertTokenizer, BertConfig
import torch

enc = BertTokenizer.from_pretrained("bert-base-uncased")

# Tokenizing input text
text = "[CLS] Who was Jim Henson ? [SEP] Jim Henson was a puppeteer [SEP]"
tokenized_text = enc.tokenize(text)

# Masking one of the input tokens
masked_index = 8
tokenized_text[masked_index] = '[MASK]'
indexed_tokens = enc.convert_tokens_to_ids(tokenized_text)
segments_ids = [0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1]

# Creating a dummy input
tokens_tensor = torch.tensor([indexed_tokens])
segments_tensors = torch.tensor([segments_ids])
dummy_input = [tokens_tensor, segments_tensors]

# Initializing the model with the torchscript flag
# Flag set to True even though it is not necessary as this model does not have an LM Head.
config = BertConfig(vocab_size_or_config_json_file=32000, hidden_size=768,
    num_hidden_layers=12, num_attention_heads=12, intermediate_size=3072, torchscript=True)

# Instantiating the model
model = BertModel(config)

# The model needs to be in evaluation mode
model.eval()

# Creating the trace
traced_model = torch.jit.trace(model, [tokens_tensor, segments_tensors])
torch.jit.save(traced_model, "traced_bert.pt")
```

And then I get an error:

```
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-10-d850721ce4cc> in <module>
     22 # Flag set to True even though it is not necessary as this model does not have an LM Head.
     23 config = BertConfig(vocab_size_or_config_json_file=32000, hidden_size=768,
---> 24     num_hidden_layers=12, num_attention_heads=12, intermediate_size=3072, torchscript=True)
     25 
     26 # Instantiating the model

TypeError: __init__() got an unexpected keyword argument 'torchscript'
```



Then I looked at source code and did not find torchscript argument in constructor. Help me please?
I realised what was the problem. There is an error in documentation.
```from pytorch_pretrained_bert import BertModel, BertTokenizer, BertConfig```
should be
```from pytorch_transformers import BertModel, BertTokenizer, BertConfig```

And also [there](https://github.com/huggingface/pytorch-transformers/blob/master/docs/source/torchscript.rst) is fixed version, but webview documentation looks outdated.<|||||>Indeed. We will update the web documentation, thanks for the report