Quickstart BERT Example: Assertion Error
## 🐛 Bug

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Model I am using (Bert, XLNet....): Bert

Language I am using the model on (English, Chinese....): English

The problem arises when:
* I run the official BERT Example in my local Jupyter Lab environment: Copy pasted the code and ran it in one cell. 

## To Reproduce

Steps to reproduce the behavior:

1. Download and install Transformers from source
2. Start Jupyter lab
3. Copy paste and run the Quickstart BERT Example

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## Expected behavior

Expected the assertion to pass.

## Environment

* OS: Oracle Linux Server 7.7
* Python version: 3.7.5
* PyTorch version: 1.3.1
* PyTorch Transformers version (or branch): master
* Using GPU ? No
* Distributed of parallel setup ? No

## Additional context

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![transformers_bug](https://user-images.githubusercontent.com/4649183/71553748-ff6e9e80-2a3e-11ea-813e-51d291d78870.png)

Not quite sure what happened. Restarted my kernel after installing packages from ```examples/requirements.txt``` and is fixed. Closing the issue.