fix learning without negate_trials

main.lua

@@ -202,7 +202,11 @@ local function prepare_epoch()
 end
 
 local function load_next_trial()
-    if cfg.negate_trials then trial_neg = not trial_neg end
+    if cfg.negate_trials then
+        trial_neg = not trial_neg
+    else
+        trial_neg = true
+    end
     trial_i = trial_i + 1
     if trial_i == 0 and not cfg.unperturbed_trial then
         trial_i = 1

nn.lua

@@ -743,6 +743,7 @@ end
 
 function Model:distribute(W)
     -- inverse operation of collect().
+    assert(W ~= nil)
     local i = 0
     for _, node in ipairs(self.nodes) do
         for _, w in ipairs(node.weights) do