def get_model_scores(pred_boxes):
    """Creates a dictionary of from model_scores to image ids.
    Args:
        pred_boxes (dict): dict of dicts of 'boxes' and 'scores'
    Returns:
        dict: keys are model_scores and values are image ids (usually filenames)
    """
    model_score = {}
    for img_id, val in pred_boxes.items():
        for score in val['scores']:
            if score not in model_score.keys():
                model_score[score] = [img_id]
            else:
                model_score[score].append(img_id)
    return model_score