"Write With Transformer" interface returning 502 on gpt2/xl model
## 🐛 Bug

The "Write With Transformer" interface is returning a `502` when the API calls the gpt2/xl model.

See: https://transformer.huggingface.co/doc/gpt2-xl

## To Reproduce

Steps to reproduce the behavior just using the API request:

```
curl 'https://transformer.huggingface.co/autocomplete/gpt2/xl' --data-binary '{"context":"See how a modern ","model_size":"gpt2/xl","top_p":0.9,"temperature":1,"max_time":1}' --compressed
```

## Expected behavior

Expecting autocomplete results, but getting this `502` response instead.
```
<html>
<head><title>502 Bad Gateway</title></head>
<body bgcolor="white">
<center><h1>502 Bad Gateway</h1></center>
<hr><center>nginx/1.14.2</center>
</body>
```

