GPT-2: 1.5B release

As the final model release of GPT-2's staged release, we're releasing the largest version (1. 5B parameters) of GPT-2 along with code and model weights to facilitate detection of outputs of GPT-2 models. While there have been larger language models released since August, we've continued with our original staged release plan in order to provide the community with a test case of a full staged release process.