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#