Fix CUDA LookupTable with 2D inputs.

The CUDA version of LookupTable was incorrect for 2D inputs. This fixes
it by viewing 2D inputs as a 1D tensor.

LookupTable.lua

@@ -55,7 +55,14 @@ function LookupTable:updateOutput(input)
 end
 
 function LookupTable:accGradParameters(input, gradOutput, scale)
-   self.gradWeight.nn.LookupTable_accGradParameters(self, self.copiedInput and self._input or input, gradOutput, scale)
+   input = self.copiedInput and self._input or input
+   if input:dim() == 2 then
+      input = input:view(-1)
+   elseif input:dim() ~= 1 then
+      error("input must be a vector or matrix")
+   end
+
+   self.gradWeight.nn.LookupTable_accGradParameters(self, input, gradOutput, scale)
 end
 
 function LookupTable:type(type)