change default reduction in xla to mean

Author: SkafteNicki

src/lightning/pytorch/strategies/xla.py

@@ -247,7 +247,9 @@ def broadcast(self, obj: TBroadcast, src: int = 0) -> TBroadcast:
 
     @override
     def reduce(
-        self, output: Union[Tensor, Any], group: Optional[Any] = None, reduce_op: Optional[Union[ReduceOp, str]] = None
+        self, 
+        output: Union[Tensor, Any], group: Optional[Any] = None, 
+        reduce_op: Optional[Union[ReduceOp, str]] = "mean"
     ) -> Tensor:
         if not isinstance(output, Tensor):
             output = torch.tensor(output, device=self.root_device)