Access denied to pretrained GPT2 model
## 🐛 Bug

<!-- Important information -->

Model I am using (Bert, XLNet....): GPT2

Language I am using the model on (English, Chinese....): English

The problem arise when using:
* [ ] the official example scripts: (give details)
* [x] my own modified scripts: I cannot load the GPT2 small pretrained model.

The tasks I am working on is:
* [ ] an official GLUE/SQUaD task: (give the name)
* [x] my own task or dataset: I am trying to instantiate a GPT2 pretrained model.

## To Reproduce

Steps to reproduce the behavior:

```
from pytorch_transformers import AutoModel
model = AutoModel.from_pretrained('gpt2')
```

This only happens with the 'gpt2' shortcut, not others ('gpt2-medium', etc.)

<!-- If you have a code sample, error messages, stack traces, please provide it here as well. -->

Error message: 
```
---------------------------------------------------------------------------
UnpicklingError                           Traceback (most recent call last)
<ipython-input-14-e057f5f0ba3e> in <module>
      1 from pytorch_transformers import AutoModel
----> 2 model = AutoModel.from_pretrained('gpt2')

~/pipeline/.venv/lib/python3.7/site-packages/pytorch_transformers/modeling_auto.py in from_pretrained(cls, pretrained_model_name_or_path, *model_args, **kwargs)
    246             return OpenAIGPTModel.from_pretrained(pretrained_model_name_or_path, *model_args, **kwargs)
    247         elif 'gpt2' in pretrained_model_name_or_path:
--> 248             return GPT2Model.from_pretrained(pretrained_model_name_or_path, *model_args, **kwargs)
    249         elif 'transfo-xl' in pretrained_model_name_or_path:
    250             return TransfoXLModel.from_pretrained(pretrained_model_name_or_path, *model_args, **kwargs)

~/pipeline/.venv/lib/python3.7/site-packages/pytorch_transformers/modeling_utils.py in from_pretrained(cls, pretrained_model_name_or_path, *model_args, **kwargs)
    537 
    538         if state_dict is None and not from_tf:
--> 539             state_dict = torch.load(resolved_archive_file, map_location='cpu')
    540         if from_tf:
    541             # Directly load from a TensorFlow checkpoint

~/pipeline/.venv/lib/python3.7/site-packages/torch/serialization.py in load(f, map_location, pickle_module, **pickle_load_args)
    385         f = f.open('rb')
    386     try:
--> 387         return _load(f, map_location, pickle_module, **pickle_load_args)
    388     finally:
    389         if new_fd:

~/pipeline/.venv/lib/python3.7/site-packages/torch/serialization.py in _load(f, map_location, pickle_module, **pickle_load_args)
    562             f.seek(0)
    563 
--> 564     magic_number = pickle_module.load(f, **pickle_load_args)
    565     if magic_number != MAGIC_NUMBER:
    566         raise RuntimeError("Invalid magic number; corrupt file?")

UnpicklingError: invalid load key, '<'.
```

Contents of the downloaded file:
```
<?xml version="1.0" encoding="UTF-8"?>
<Error><Code>AccessDenied</Code><Message>Access Denied</Message><RequestId>D027DE3363BB3D26</RequestId><HostId>GPDcAN+fZerpFZ5ZR9ZnATk3XIJ4GgLjCDMLnzvs48MRKG8soooyb8HM+zjBA0Gnn7HJc4CRqpA=</HostId></Error>%
```

## Expected behavior

Successfully load the pretrained model.

## Environment

* OS: macOS Catalina
* Python version: 3.7.4
* PyTorch version: 1.1.0
* PyTorch Transformers version (or branch): 1.1.0
* Using GPU ? No
* Distributed of parallel setup ? No
* Any other relevant information:

## Additional context

<!-- Add any other context about the problem here. -->

I'm having the same error<|||||>There is a known (temporary) issue with our `gpt2` model – can you guys use `gpt2-medium` or `distilgpt2` instead for now?

cc @LysandreJik @thomwolf @n1t0 @clmnt <|||||>Sure thing! Thanks for letting us know :)<|||||>(should be fixed now)<|||||>Thank you!<|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
