change ensure_ascii to False for JsonChatStr (#2691)

lm_eval/models/api_models.py

@@ -265,7 +265,7 @@ class TemplateAPI(TemplateLM):
             )
         else:
             # bit of a hack. We'll load back before sending to the API
-            return JsonChatStr(json.dumps(chat_history))
+            return JsonChatStr(json.dumps(chat_history, ensure_ascii=False))
 
     @cached_property
     def eot_token_id(self) -> Optional[int]: