Update teardown for TPU acc (#7211)

pytorch_lightning/accelerators/tpu.py

@@ -50,6 +50,9 @@ class TPUAccelerator(Accelerator):
             raise MisconfigurationException("TPUs only support a single tpu core or tpu spawn training.")
         return super().setup(trainer, model)
 
+    def teardown(self) -> None:
+        pass
+
     def run_optimizer_step(
         self, optimizer: Optimizer, optimizer_idx: int, lambda_closure: Callable, **kwargs: Any
     ) -> None: