Add `max_length` to model config

This is required by the conversational pipeline.

config.json

@@ -32,5 +32,6 @@
   "torch_dtype": "float32",
   "transformers_version": "4.22.1",
   "use_cache": true,
-  "vocab_size": 64000
+  "vocab_size": 64000,
+  "max_length": 2048
 }