fix(train): fix an error in computing layer's error

feat(examples): add an example (xor)

app/Main.hs

@@ -9,19 +9,16 @@ import Debug.Trace
 -- 2x3 + 1x3
 -- 3x1 + 1x1
 
-main :: IO ()
+-- main :: IO [()]
 main =
-  let learning_rate = 0.01
-      ih = randomLayer 0 (2, 10)
-      ho = randomLayer 1 (10, 1)
+  let learning_rate = 0.5
+      ih = randomLayer 0 (2, 8)
+      ho = randomLayer 1 (8, 1)
       network = ih :- O ho
-      inputs = [vector [0, 1], vector [1, 1], vector [1, 0], vector [0, 0]]
 
-      -- result = forward input network
+      inputs = [vector [0, 1], vector [1, 0], vector [1, 1], vector [0, 0]]
+      labels = [vector [1], vector [1], vector [0], vector [0]]
 
-      labels = [vector [1], vector [0], vector [1], vector [0]]
-
-      updated_network = session inputs network labels learning_rate 100
-      -- updated_network = train (head inputs) network (head labels) 0.5
+      updated_network = session inputs network labels learning_rate (2, 1000)
       results = map (`forward` updated_network) inputs
   in print results