add constructor for booster
add constructor for booster

huanzhang12/lightgbm-gpu,cbecker/LightGBM,Laurae2/LightGBM,microsoft/LightGBM,henry0312/LightGBM,henry0312/LightGBM,sightmachine/LightGBM,sightmachine/LightGBM,Allardvm/LightGBM,sightmachine/LightGBM,olofer/LightGBM,cbecker/LightGBM,huanzhang12/LightGBM,microsoft/LightGBM,Laurae2/LightGBM,huanzhang12/LightGBM,fstonezst/LightGBM,microsoft/LightGBM,cbecker/LightGBM,huanzhang12/lightgbm-gpu,cbecker/LightGBM,wxchan/LightGBM,olofer/LightGBM,huanzhang12/lightgbm-gpu,Laurae2/LightGBM,olofer/LightGBM,wxchan/LightGBM,fstonezst/LightGBM,henry0312/LightGBM,huanzhang12/LightGBM,wxchan/LightGBM,henry0312/LightGBM,Allardvm/LightGBM,Laurae2/LightGBM,sightmachine/LightGBM,wxchan/LightGBM,microsoft/LightGBM,microsoft/LightGBM,Allardvm/LightGBM,huanzhang12/lightgbm-gpu,huanzhang12/LightGBM,Allardvm/LightGBM,huanzhang12/LightGBM,Laurae2/LightGBM,huanzhang12/lightgbm-gpu,fstonezst/LightGBM,fstonezst/LightGBM,olofer/LightGBM,wxchan/LightGBM,fstonezst/LightGBM,henry0312/LightGBM,Allardvm/LightGBM,olofer/LightGBM