Always propagate shardings on kAllReduce.
Always propagate shardings on kAllReduce.

Before this change, it would ignore partial manual sharding for all-reduce
assigning fullly-replicated sharding to the all-reduce. This fully-replicated
sharding would error out later in SpmdPartitioningVisitor::HandleAllReduce.

After this change, it properly assigns manual shardings to all-reduces allowing
all-reduces which are only partially manual to be emitted.
