 keep after NMS.
        iou_thresh (float): minimum IoU between the anchor and the GT box so that they can be
            considered as positive during training.
        topk_candidates (int): Number of best detections to keep before NMS.
        positive_fraction (float): a number between 0 and 1 which indicates the proportion of positive
            proposals used during the training of the classification head. It is used to estimate the negative to
            positive ratio.
    )