add default system prompt

gptx_tokenizer.py

@@ -459,4 +459,5 @@ class SPTokenizer(HFGPTXTokenizer):
         self.chat_template = {
             lang: f"System: {sys_msg}" + "{{- '\\n'}}\n" + chat_template
             for lang, sys_msg in self.system_messages_by_lang.items()
-        }
+        }
+        self.chat_template['default'] = f"System: {self.system_messages_by_lang['EN']}" + "{{- '\\n'}}\n" + chat_template