Update demo.lua

demo.lua

@@ -21,7 +21,7 @@ print(net)
 local sound = audio.load('demo.mp3')
 
 -- data preprocessing
-if sound:size(2) > 1 then sound = sound:select(2,1) end -- select first channel (mono)
+if sound:size(2) > 1 then sound = sound:select(2,1):clone() end -- select first channel (mono)
 sound:mul(2^-23)                                        -- make range [-256, 256]
 sound = sound:view(1, 1, -1, 1)                         -- shape to BatchSize x 1 x DIM x 1
 sound = sound:cuda()                                    -- ship to GPU