Combine calls using batched Ensemble approach.

def combine_calls_parallel(samples, run_parallel):
    """Combine calls using batched Ensemble approach.
    """
    batch_groups, extras = _group_by_batches(samples, _has_ensemble)
    out = []
    if batch_groups:
        processed = run_parallel("combine_calls", ((b, xs, xs[0]) for b, xs in batch_groups.items()))
        for batch_id, callinfo in processed:
            for data in batch_groups[batch_id]:
                data["variants"].insert(0, callinfo)
                out.append([data])
    return out + extras