Add support for any GPT-2 model hosted in Huggingface

[cahya-wirawan]

Patch description
As mentioned in #4357, the current Huggingface agent only supports few english GPT2 models (small, medium, large, xl and distilgpt2), therefore I add the functionality to specify arbitrary GPT-2 model hosted in Huggingface such as anonymous-german-nlp/german-gpt2 or indonesian-nlp/gpt2.

Testing steps
Following is an example to fine tune indonesian-nlp/gpt2 model with convai2 dataset:

parlai train_model -m hugging_face/gpt2 --model_name indonesian-nlp/gpt2 --add-special-tokens True --add-start-token True -t convai2 -bs 2 -mf <modelfile>

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[stephenroller]

can we provide a default here that would fall back to the existing behavior?

[cahya-wirawan]

Actually, it is already the case. If we don't use the argument "--model_name" at all, it will fall back to the old behavior. This is because the default value for this argument is None

[stephenroller]

Cool! seems like a nice extension. Flagging for @jxmsML to also review please.

[cahya-wirawan]

Hi @stephenroller , sorry I am not familiar with PR procedure of this repo yet, I just don't know about the error in ci/circleci: unittests_38 above/below, should I create a unit test for the additional argument? and if yes, where should I put it? Thanks.

[stephenroller]

Our CircleCI is broken right now. I think this PR is fine.

[stephenroller]

Last thing: can you run "black" on the code?

[cahya-wirawan]

I did it before, but here again:
% black parlai/agents/hugging_face
All done! ✨ 🍰 ✨
6 files left unchanged.

[stephenroller]

Thanks!