message: Add documentation saying batch_reduce is faster than multiple `reduce`s in some cases.
diff: @@ -287,6 +287,9 @@ class CrossDeviceOps(object):
  Reduce each first element in `value_destination_pairs` to each second
  element which indicates the destinations.
  + This can be faster than multiple individual `reduce`s because we can
  + fuse several tensors into one or multiple packs before reduction.
  +
  Args:
  reduce_op: An instance of `tf.distribute.ReduceOp` that indicates how
  the `per_replica_value` will be reduced.
  