def get_min_max_levels():
    """The min and max FPN levels required for supporting RPN and/or RoI
    transform operations on multiple FPN levels.
    """
    min_level = LOWEST_BACKBONE_LVL
    max_level = HIGHEST_BACKBONE_LVL
    if cfg.FPN.MULTILEVEL_RPN and not cfg.FPN.MULTILEVEL_ROIS:
        max_level = cfg.FPN.RPN_MAX_LEVEL
        min_level = cfg.FPN.RPN_MIN_LEVEL
    if not cfg.FPN.MULTILEVEL_RPN and cfg.FPN.MULTILEVEL_ROIS:
        max_level = cfg.FPN.ROI_MAX_LEVEL
        min_level = cfg.FPN.ROI_MIN_LEVEL
    if cfg.FPN.MULTILEVEL_RPN and cfg.FPN.MULTILEVEL_ROIS:
        max_level = max(cfg.FPN.RPN_MAX_LEVEL, cfg.FPN.ROI_MAX_LEVEL)
        min_level = min(cfg.FPN.RPN_MIN_LEVEL, cfg.FPN.ROI_MIN_LEVEL)
    return min_level, max_level
The min and max FPN levels required for supporting RPN and/or RoI
    transform operations on multiple FPN levels.
    
detectron/modeling/FPN.py