README: wrong model class in example given
Author: FilipposVentirozos
Pull request: yes

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Fixes # 

Just a small typo in the README. It had another model-class which did not work. You can view inside the `run_generation.py` that the suggested `--model_type` does not exist. It was a simple change from `openai-community/gpt2` to `gpt2`

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Comments:
Hi @FilipposVentirozos, thanks for opening a PR! 

The model [openai-community/gpt2](https://huggingface.co/openai-community/gpt2) does exist. Are you experiencing an error trying to run the examples? 

Hello @amyeroberts, thank you for responding.

Yes, I cannot reproduce with the README example.

Specifically, the README talks about how to use the `run_generation.py` (Which might be outdated, I think the `run_generation_contrastive_search.py` encompasses it, anyways that's another topic).

The `run_generation.py`  it shown to take two variables:
--model_type=openai-community/gpt2 \
--model_name_or_path=openai-community/gpt2

The second one is correct. But the first one (i.e. `model_type`) is incorrect. Because in the `run_generation.py` in it's model's dictionary it does not exist. Have a look here

https://github.com/huggingface/transformers/blob/d71f5b3ea8b4a3a530d0c868e87d57311b177c1c/examples/pytorch/text-generation/run_generation.py#L64


@FilipposVentirozos Ah, I see. Yes, in this case we still want just `gpt2`. In the PR diff, there's changes to a lot of unrelated files. Could you remove these? Once that's done we should be good to merge