A convenient script to switch visible Nvidia GPU.
wget https://raw.githubusercontent.com/lanpa/usegpu/master/appendtobash && cat appendtobash>>~/.bashrc
source ~/.bashrc
usegpu 1 # use gpu 1
usegpu 1,2 #usegpu 1 and 2
usegpu 0,2 #usegpu 0 and 2
usegpu #show current visible gpu