fix softmax

nn.lua

@@ -2,6 +2,7 @@ local ceil = math.ceil
 local cos = math.cos
 local exp = math.exp
 local floor = math.floor
+local huge = math.huge
 local insert = table.insert
 local ipairs = ipairs
 local log = math.log
@@ -26,6 +27,7 @@ local function helpme() print(debug.traceback('helpme', 2):gsub("\n", "\r\n")) e
 -- general utilities
 
 local function copy(t, out) -- shallow copy
+    assert(type(t) == "table")
     local out = out or {}
     for k, v in pairs(t) do out[k] = v end
     return out
@@ -559,7 +561,7 @@ function Softmax:forward(X)
     if self.bs ~= bs then self:reset_cache(bs) end
     local Y = self.cache
 
-    local alpha = 0
+    local alpha = -huge
     local num = {} -- TODO: cache
     local den = 0