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]>

Author: liob

pytorch_lightning/plugins/training_type/horovod.py

@@ -150,7 +150,7 @@ def reduce(self, tensor, group: Optional[Any] = None, reduce_op: Optional[Union[
 
         if reduce_op in (None, "avg", "mean"):
             reduce_op = hvd.Average
-        elif reduce_op == "sum":
+        elif reduce_op in ("sum", ReduceOp.SUM):
             reduce_op = hvd.Sum
         else:
             raise ValueError(f"unrecognized `reduce_op`: {reduce_op}")