docs: public String validateLayout(Set<Integer> liveHosts) {
          if (m_unbalancedPartitionCount > 0) {
              return String.format("%d out of %d partitions are unbalanced across placement groups.",
                      m_unbalancedPartitionCount, partitionsById.size());
          }
  
          if (liveHosts == null) {
              return null;
          }
  
          // verify the partition leaders on live hosts
          for (Host host : hostsById.values()) {
              if (liveHosts.contains(Integer.valueOf(host.id))) {
                  for (Partition p : host.partitions) {
                      if (!liveHosts.contains(Integer.valueOf(p.leaderHostId))) {
                          return String.format("The leader host %d of partition %d is not on live host.",
                                  p.leaderHostId, p.id);
                      }
                  }
              }
          }
          return null;
      }