Add customizable ban tokens (#3899)

api-examples/api-example-stream.py

@@ -53,6 +53,7 @@ async def run(context):
         'add_bos_token': True,
         'truncation_length': 2048,
         'ban_eos_token': False,
+        'custom_token_bans': '',
         'skip_special_tokens': True,
         'stopping_strings': []
     }