fix: Add missing shutdown handler for TorchtunePostTrainingImpl

Without it, when server shuts down, it posts the following warning:

__main__:129 server: No shutdown method for TorchtunePostTrainingImpl

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>

llama_stack/providers/inline/post_training/torchtune/post_training.py

@@ -43,6 +43,9 @@ class TorchtunePostTrainingImpl:
         self.jobs = {}
         self.checkpoints_dict = {}
 
+    async def shutdown(self):
+        pass
+
     async def supervised_fine_tune(
         self,
         job_uuid: str,