why need opencl if I user 'th train.lua --cuda --gpu 0'

[rzai]

root@rzai00:~/prj/neuralconvo# th train.lua --cuda --gpu 0 /home/rzai/torch/install/bin/luajit: /home/rzai/torch/install/share/lua/5.1/trepl/init.lua:384: /home/rzai/torch/install/share/lua/5.1/trepl/init.lua:384: module 'cltorch' not found:No LuaRocks module found for cltorch no field package.preload['cltorch'] no file '/home/rzai/.luarocks/share/lua/5.1/cltorch.lua' no file '/home/rzai/.luarocks/share/lua/5.1/cltorch/init.lua' no file '/home/rzai/torch/install/share/lua/5.1/cltorch.lua' no file '/home/rzai/torch/install/share/lua/5.1/cltorch/init.lua' no file './cltorch.lua' no file '/home/rzai/torch/install/share/luajit-2.1.0-beta1/cltorch.lua' no file '/usr/local/share/lua/5.1/cltorch.lua' no file '/usr/local/share/lua/5.1/cltorch/init.lua' no file '/home/rzai/.luarocks/lib/lua/5.1/cltorch.so' no file '/home/rzai/torch/install/lib/lua/5.1/cltorch.so' no file '/home/rzai/torch/install/lib/cltorch.so' no file './cltorch.so' no file '/usr/local/lib/lua/5.1/cltorch.so' no file '/usr/local/lib/lua/5.1/loadall.so' stack traceback: [C]: in function 'error' /home/rzai/torch/install/share/lua/5.1/trepl/init.lua:384: in function 'require' train.lua:1: in main chunk [C]: in function 'dofile' ...rzai/torch/install/lib/luarocks/rocks/trepl/scm-1/bin/th:145: in main chunk [C]: at 0x00406670 root@rzai00:~/prj/neuralconvo#