Add max_new_tokens to the config

To avoid the warning
```
UserWarning: Neither `max_length` nor `max_new_tokens` has been set, `max_length` will default to 20 (`self.config.max_length`). Controlling `max_length` via the config is deprecated and `max_length` will be removed from the config in v5 of Transformers -- we recommend using `max_new_tokens` to control the maximum length of the generation.
warnings.warn
```

config.json

@@ -18,6 +18,7 @@
   "output_past": true,
   "pad_token_id": 0,
   "relative_attention_num_buckets": 32,
+  "max_new_tokens": 25,
   "task_specific_params": {
     "summarization": {
       "early_stopping": true,