public Map<String, List<TopicPartition>> groupIdsToPartitions() {
        Map<String, List<TopicPartition>> groupIdsToPartitions = new HashMap<>();
        for (OffsetFetchRequestGroup group : data.groups()) {
            List<TopicPartition> tpList = null;
            if (group.topics() != ALL_TOPIC_PARTITIONS_BATCH) {
                tpList = new ArrayList<>();
                for (OffsetFetchRequestTopics topic : group.topics()) {
                    for (Integer partitionIndex : topic.partitionIndexes()) {
                        tpList.add(new TopicPartition(topic.name(), partitionIndex));
                    }
                }
            }
            groupIdsToPartitions.put(group.groupId(), tpList);
        }
        return groupIdsToPartitions;
    }
Indicates that it is not possible to fetch consumer groups in batches with FetchOffset.
Instead consumer groups' offsets must be fetched one by one.
clients/src/main/java/org/apache/kafka/common/requests/OffsetFetchRequest.java