Generic Huggingface GPT-2 model

[cahya-wirawan]

Hi,
I am trying to finetune Huggingface gpt2 model for my own language. As I understand from the source code of hugggingface agent, the model that can be used is limited only to few standard english gpt-2 models (small, medium, large and xl). There is no possibility to use other gpt2 models. Therefore I add this functionality cahya-wirawan@6b081c2 , where I add a new argument (--model_name) to set any existing Huggingface gpt-2 models. If this functionality is not done yet before (I am new in ParlAI, so maybe I overlook it) and if it could be helpful for other, I will create a PR.

[klshuster]

Yes, please submit a PR!