do not save sparse_threshold_ in FeatureGroup
do not save sparse_threshold_ in FeatureGroup

huanzhang12/lightgbm-gpu,huanzhang12/lightgbm-gpu,huanzhang12/LightGBM,huanzhang12/LightGBM,huanzhang12/LightGBM,huanzhang12/lightgbm-gpu,huanzhang12/lightgbm-gpu,huanzhang12/LightGBM,huanzhang12/LightGBM,huanzhang12/lightgbm-gpu