fix type annotation for debug arg (#24033)

* fix type annotation for debug arg

* fix TypeErorr

src/transformers/training_args.py

@@ -873,7 +873,7 @@ class TrainingArguments:
             )
         },
     )
-    debug: str = field(
+    debug: Union[str, List[DebugOption]] = field(
         default="",
         metadata={
             "help": (
@@ -1563,8 +1563,11 @@ class TrainingArguments:
             )
             self.debug += " tpu_metrics_debug"
             self.tpu_metrics_debug = False
+
         if isinstance(self.debug, str):
             self.debug = [DebugOption(s) for s in self.debug.split()]
+        elif self.debug is None:
+            self.debug = []
 
         self.deepspeed_plugin = None
         if self.deepspeed: