public List<TopicPartition> partitions() {
        if (isAllPartitions()) {
            return null;
        }
        List<TopicPartition> partitions = new ArrayList<>();
        for (OffsetFetchRequestTopic topic : data.topics()) {
            for (Integer partitionIndex : topic.partitionIndexes()) {
                partitions.add(new TopicPartition(topic.name(), partitionIndex));
            }
        }
        return partitions;
    }
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