Fix Checkpoint issue when using Horovod distributed backend (PyTorchLightning#6947) (#6958)

Co-Authored-By: Adrian Wälchli <[email protected]>

Co-authored-by: Adrian Wälchli <[email protected]>

(cherry picked from commit b37b58a)

Author: liob

pytorch_lightning/plugins/training_type/horovod.py

@@ -136,7 +136,9 @@ def reduce(self, output, group: Optional[Any] = None, reduce_op: Optional[Union[
                 "Unset `group`."
             )
 
-        if reduce_op is None or reduce_op == "sum":
+        if reduce_op in (None, "avg", "mean"):
+            reduce_op = hvd.Average
+        elif reduce_op in ("sum", ReduceOp.SUM):
             reduce_op = hvd.Sum
         elif isinstance(reduce_op, str) and reduce_op in ("avg", "mean"):
             reduce_op = hvd.Average