Create backward method with updateGradInput

gmodule.lua

@@ -479,6 +479,78 @@ function gModule:accGradParameters(input,gradOutput,lr)
    end
 end
 
+function gModule:backward(input,gradOutput,scale)
+   local function neteval(node)
+      if node.data.selectindex then
+         assert(not node.data.module, "the selectindex-handling nodes should have no module")
+         assert(#node.children == 1, "only the splitted node should be the input")
+         local child = node.children[1]
+         local go = getTotalGradOutput(node)
+         child.data.gradOutput = child.data.gradOutput or {}
+         assert(#child.data.gradOutput <= 1, "the splitted node should be used only once")
+         -- The data.gradOutput holds the to-be-summed gradients.
+         child.data.gradOutput[1] = child.data.gradOutput[1] or {}
+         assert(not child.data.gradOutput[1][node.data.selectindex], "no gradOutput should be assigned yet")
+         child.data.gradOutput[1][node.data.selectindex] = go
+      else
+         local gradOutput = getTotalGradOutput(node)
+         -- backward through this node
+         -- If no module is present, the node behaves like nn.Identity.
+         local gradInput
+         if not node.data.module then
+            gradInput = gradOutput
+         else
+            local input = node.data.input
+            -- a parameter node is captured
+            if input == nil and node.data.module ~= nil then
+               input = {}
+            end
+            if #input == 1 then
+               input = input[1]
+            end
+            local module = node.data.module
+            gradInput = module:backward(input,gradOutput,scale)
+         end
+         -- propagate the output to children
+         for i,child in ipairs(node.children) do
+            child.data.gradOutput = child.data.gradOutput or {}
+            local mapindex = node.data.mapindex[child.data]
+            local gi
+            if #node.children == 1 then
+               gi = gradInput
+            else
+               gi = gradInput[mapindex]
+            end
+            table.insert(child.data.gradOutput,gi)
+         end
+      end
+      if self.verbose then
+         print(' V : ' .. node:label())
+      end
+   end
+   local outnode = self.outnode
+   if #outnode.children > 1 and #gradOutput ~= #outnode.children then
+      error(string.format('Got %s gradOutputs instead of %s', #gradOutput, #outnode.children))
+   end
+   for _,node in ipairs(self.backwardnodes) do
+      local gradOutput = node.data.gradOutput
+      while gradOutput and #gradOutput >0 do
+         table.remove(gradOutput)
+      end
+   end
+   -- Set the starting gradOutput.
+   outnode.data.gradOutput = outnode.data.gradOutput or {}
+   outnode.data.gradOutput[1] = gradOutput
+
+   for i,node in ipairs(self.backwardnodes) do
+      neteval(node)
+   end
+
+   assert(#self.innode.data.gradOutput == 1, "expecting the innode to be used only once")
+   self.gradInput = self.innode.data.gradOutput[1]
+   return self.gradInput
+end
+
 function gModule:read(file)
    local data = file:readObject()
    for k, v in pairs(data) do