Bug in language_modeling.py calling tokenizer.num_special_tokens_to_add
## Environment info
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- `transformers` version: 3.0.2
- Platform: Linux-4.4.0-1081-aws-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.6.9
- PyTorch version (GPU?): 1.6.0+cu101 (True)
- Tensorflow version (GPU?): 2.1.0 (True)
- Using GPU in script?: yes
- Using distributed or parallel set-up in script?: no

### Who can help
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 tokenizers: @mfuntowicz
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## Information

Model I am using GPT-2

The class TextDataset call tokenizer.num_special_tokens_to_add(pair=False) but the correct argument is called is_pair. I assume the bugfix corresponds to transformers repo.

## To reproduce

https://github.com/huggingface/transformers/blob/e49393c3617e877f0370f7bad7c7e823808c5bfb/src/transformers/data/datasets/language_modeling.py#L27

https://github.com/huggingface/tokenizers/blob/master/bindings/python/tokenizers/implementations/base_tokenizer.py#L20


I'm using transformers 3.0.2 and tokenizers 0.8.1rc1 (try to update but says its incompatible)

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## Expected behavior

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I was using a wrong approch, it works if I train the tokenizer, save de params, and load into a FastTokenizer impl