Update generation_config.json

With the recent `transformers` this generates:
```
The following generation flags are not valid and may be ignored: ['pad_token_id']. Set `TRANSFORMERS_VERBOSITY=info` for more details.
```
so proposing to remove it? probably in the rest of the hf testing models as well?

generation_config.json

@@ -2,6 +2,5 @@
   "_from_model_config": true,
   "bos_token_id": 0,
   "eos_token_id": 1,
-  "pad_token_id": -1,
   "transformers_version": "4.38.0.dev0"
 }