Enable a soft ordering for collectives which adds dependencies as node attrs.

Before this change, OrderCollectives would add control edges between collective
ops to make their execution sequential.  This change introduces a technique of
encoding the ordering information as a node attribute.  In future changes, the
collective executor will parse these changes and enforce a fine-grained
ordering between collective ops.

Additionally, this change also
* restricts ordering to CollectiveReduce nodes,
* and fixes a bug so that this ordering is invoked from both DirectSession and
MasterSession.

PiperOrigin-RevId: 228801501