src/transformers/data/metrics/squad_metrics.py/find_best_thresh_v2
def find_best_thresh_v2(preds, scores, na_probs, qid_to_has_ans):
    num_no_ans = sum(1 for k in qid_to_has_ans if not qid_to_has_ans[k])
    cur_score = num_no_ans
    best_score = cur_score
    best_thresh = 0.0
    qid_list = sorted(na_probs, key=lambda k: na_probs[k])
    for i, qid in enumerate(qid_list):
        if qid not in scores:
            continue
        if qid_to_has_ans[qid]:
            diff = scores[qid]
        else:
            if preds[qid]:
                diff = -1
            else:
                diff = 0
        cur_score += diff
        if cur_score > best_score:
            best_score = cur_score
            best_thresh = na_probs[qid]

    has_ans_score, has_ans_cnt = 0, 0
    for qid in qid_list:
        if not qid_to_has_ans[qid]:
            continue
        has_ans_cnt += 1

        if qid not in scores:
            continue
        has_ans_score += scores[qid]

    return 100.0 * best_score / len(scores), best_thresh, 1.0 * has_ans_score / has_ans_cnt