Adding gpt-2 large (774M parameters) model
Per request #1061

Also, fix a small restriction in a few conversion scripts (easier loading from original JSON configuration files).
Oops @LysandreJik forgot to add it in the [list of pretrained models of the doc](https://huggingface.co/pytorch-transformers/pretrained_models.html)<|||||>@thomwolf Added it with 2f93971
<|||||>You're the best!