Add str to TrainingArguments report_to type hint (#30078)

* Add str to TrainingArguments report_to type hint

* Swap order in Union

* Merge Optional into Union

https://github.com/huggingface/transformers/pull/30078#issuecomment-2042227546

src/transformers/training_args.py

@@ -1181,7 +1181,7 @@ class TrainingArguments:
         default="length",
         metadata={"help": "Column name with precomputed lengths to use when grouping by length."},
     )
-    report_to: Optional[List[str]] = field(
+    report_to: Union[None, str, List[str]] = field(
         default=None, metadata={"help": "The list of integrations to report the results and logs to."}
     )
     ddp_find_unused_parameters: Optional[bool] = field(