Error while fine-tuning model for GPT2
## 🐛 Bug

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Model I am using GPT2:

Language I am using the model on English:

The problem arise when using:
* [ ] the official example scripts: I run the run_lm_finetuning.py script from the examples folder

The tasks I am working on is:
* [ ] my own task or dataset: The WritingPrompts dataset.

## To Reproduce

Steps to reproduce the behavior:

1. Running the run_lm_generation.py on the WP dataset. 
 python run_lm_finetuning.py --output_dir=output_ft_gpt2 --model_type=gpt2 --model_name_or_path=gpt2 --do_train --train_data_file=../stories_data/writingPrompts/train.wp_target --do_eval --eval_data_file=../stories_data/writingPrompts/test.wp_target --block_size=128 --save_total_limit=100


Error: Traceback (most recent call last):
  File "run_lm_finetuning.py", line 538, in <module>
    main()
  File "run_lm_finetuning.py", line 485, in main
    train_dataset = load_and_cache_examples(args, tokenizer, evaluate=False)
  File "run_lm_finetuning.py", line 97, in load_and_cache_examples
    dataset = TextDataset(tokenizer, file_path=args.eval_data_file if evaluate else args.train_data_file, block_size=args.block_size)
  File "run_lm_finetuning.py", line 80, in __init__
    self.examples.append(tokenizer.build_inputs_with_special_tokens(tokenized_text[i:i+block_size]))
AttributeError: 'GPT2Tokenizer' object has no attribute 'build_inputs_with_special_tokens'




Hi! It seems like you have taken the example from the latest version but that your library is not up to date. Could you tell me the version of your `transformers` library ?<|||||>Hi! 
I am using the version 2.0.0.<|||||>If you´re using the version 2.0.0 you should use the [script that was used in this version](https://github.com/huggingface/transformers/blob/v2.0.0/examples/run_lm_finetuning.py). The current script works on version 2.1!<|||||>Thanks!