Add RoBERTa-based GPT-2 Output Detector from OpenAI
converted from https://github.com/openai/gpt-2-output-dataset/tree/master/detector
Co-Authored-By: Lysandre Debut <lysandre.debut@reseau.eseo.fr>
Co-Authored-By: Jong Wook Kim <jongwook@nyu.edu>
Co-Authored-By: Jeff Wu <wuthefwasthat@gmail.com>
