Masked tokens are -1 not -100?
## 🐛 Bug

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

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

The problem arise when using:
* [X] the official example scripts: (give details)
* [ ] my own modified scripts: (give details)

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [X] my own task or dataset: (give details)

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## Expected behavior

#2130

After haveing updated run_lm_finetuning.py to the newest git branch, I have incountered an error in train(). Haveing spent some time trying to figure it out, I realized, that masked tokens have been changed from -1 to -100. If I change it back to -1 it all works again.

https://github.com/huggingface/transformers/blob/f599623a99b808e3d5926d89cd13237457b9eeba/examples/run_lm_finetuning.py#L179

Won't work:
`
labels[~masked_indices] = -100  # We only compute loss on masked tokens
`

Works:
`
labels[~masked_indices] = -1  # We only compute loss on masked tokens
`

<!-- A clear and concise description of what you expected to happen. -->

## Environment

* OS:
* Python version:
* PyTorch version:
* PyTorch Transformers version (or branch):
* Using GPU ?
* Distributed or parallel setup ?
* Any other relevant information:

## Additional context

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To fast... Sorry :)
https://github.com/huggingface/transformers/issues/2442