Fix the new stop_token_ids argument (#1558)

We forgot the default somewhere

Author: mattdangerw

keras_nlp/models/causal_lm.py

@@ -299,7 +299,7 @@ def generate(
         self,
         inputs,
         max_length=None,
-        stop_token_ids=None,
+        stop_token_ids="auto",
     ):
         """Generate text given prompt `inputs`.