GPT2 774M weights released!
## 🚀 Feature

Hi! OpenAI released the 774M weights in GPT2, is it possible to integrate this into pytorch-transformers?

https://twitter.com/OpenAI/status/1163843803884601344

Also, sorry for the obnoxiously quick ask! Thanks for all the great work you do for the community.

Thanks!
I did the following:

1. Run `download_model.py 774` from [here](https://github.com/openai/gpt-2)
2. Create a file named `config.json` with the following contents (Might be correct but I am not super sure):
```json
{
    "vocab_size": 50257,
    "n_ctx": 1024,
    "n_embd": 1280,
    "n_head": 20,
    "n_layer": 36,
    "n_positions": 1024,
    "embd_pdrop":0.1,
    "attn_pdrop": 0.1,
    "resid_pdrop": 0.1,
    "layer_norm_epsilon": 1e-5,
    "initializer_range": 0.02
}
```

3. Clone this repo

4. Run ```python .\pytorch-transformers\pytorch_transformers\convert_gpt2_checkpoint_to_pytorch.py --gpt2_checkpoint_path models/774M --pytorch_dump_folder_path ./ --gpt2_config_file config.json```

5. Use it with 
```
config = GPT2Config.from_pretrained("config.json")
model = GPT2LMHeadModel.from_pretrained("pytorch_model.bin", config=config)
```

6. Realize there's no way you can fine-tune this your PC's GPU you need to rent something with more memory.<|||||>We've added it on master.
You can install from source and use the shortcut name `gpt2-large` to use it (but beware, it's big!)<|||||>Question: Will the gpt2-large be added to Write With Transformer? I've been eagerly looking forward to that since the moment the 774M was released!<|||||>@zacharymacleod Glad you asked! We're definitely planning on adding it in the near future :)<|||||>Seems to me as if this has been addressed via #1064 . Closing the feature request now!