No CORS Policy for Write With Transformer Endpoints
# 🐛 Bug

## Information
Write with Transformer is a very good place for trying available models. But, there is no CORS policy to secure it. This could potentially be used by an unauthorized third-parties or attackers to DDoS.

## To reproduce

Steps to reproduce the behavior:
POST request to `/autocomplete/distilgpt2/small`
```
curl -d '{"context":"See how a modern neural network auto-completes your text 🤗","model_size":"distilgpt2/small","top_p":0.9,"temperature":1,"max_time":1}' -H 'Content-Type: application/json' https://transformer.huggingface.co/autocomplete/distilgpt2/small
```
Potential endpoints:
1. `/autocomplete/distilgpt2/small`
1. `/autocomplete/gpt2/small`
1. `/autocomplete/gpt2/medium`
1. `/autocomplete/gpt2/large`
1. `/autocomplete/gpt2/xl` <- 504 error
1. `/autocomplete/pplm` <- 504 error


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## Expected behavior

<!-- A clear and concise description of what you would expect to happen. -->

The endpoints could be used by unauthorized third-parties and risk of DDoS .
Hi @ivokun – this was actually on purpose as someone was using it as an API

We'll secure it down the line if it gets abused.