The default max tokens of 215 is too small, answers are often cut off.I will modify it to 512 to address this issue. (#845)

### What problem does this PR solve?

### Type of change

- [x] Refactoring

api/db/db_models.py

@@ -759,7 +759,7 @@ class Dialog(DataBaseModel):
         help_text="English|Chinese")
     llm_id = CharField(max_length=128, null=False, help_text="default llm ID")
     llm_setting = JSONField(null=False, default={"temperature": 0.1, "top_p": 0.3, "frequency_penalty": 0.7,
-                                                 "presence_penalty": 0.4, "max_tokens": 215})
+                                                 "presence_penalty": 0.4, "max_tokens": 512})
     prompt_type = CharField(
         max_length=16,
         null=False,