Internal server error when echo is true.

```
curl http://localhost:8000/v1/completions -H "Content-Type: application/json" -d '{"model": "google/gemma-7b-it", "prompt": "<start_of_turn>user\nHello<end_of_turn>\n<start_of_turn>model\n", "max_tokens": 4, "echo": true, "stream": false}'                                                                               
Internal Server Error
```