Make 'mean' doubly differentiable (#241)

Use stopGradient on shape computations
Add opGrad for StopGradient

tensorflow-ops/src/TensorFlow/Gradient.hs

@@ -574,7 +574,7 @@ opGrad "Sum" _ [toT -> x, toT -> indices] [dz] =
     grad = reshape dz outputShapeKeptDims
 
 opGrad "Mean" u v@[toT -> x, _] w =
-    [Just $ dz `CoreOps.div` CoreOps.cast factor, Nothing]
+    [Just $ dz `CoreOps.div` (CoreOps.stopGradient $ CoreOps.cast $ factor), Nothing]
   where
     [Just dz, Nothing] = opGrad "Sum" u v w
     inputShape = shape (x :: Tensor Build a)
@@ -858,6 +858,7 @@ opGrad "Fill" _ _ [dz] = [Nothing, Just $ sum dz rx]
 opGrad "ReadVariableOp" _ _ [dz] = [Just $ expr dz]
 
 opGrad "Const" _ _ _ = [Nothing, Nothing]
+opGrad "StopGradient" _ _ _ = [Nothing]
 opGrad "VarHandleOp" _ _ _ = []
 
 opGrad "Sqrt" _ [toT -> x] [dz] = [Just $ sq' `CoreOps.mul` dz]
@@ -901,6 +902,7 @@ numOutputs o =
         "Neg" -> 1
         "Pad" -> 1
         "Placeholder" -> 1
+        "StopGradient" -> 1
         "OneHot" -> 1
         "ReadVariableOp" -> 1
         "RefIdentity" -> 1

tensorflow-ops/tests/GradientTest.hs

@@ -230,6 +230,23 @@ testAddNGradient = testCase "testAddNGradient" $ do
         TF.gradients y [x] >>= TF.run
     V.fromList [2, 2, 2 :: Float] @=? dx
 
+testMeanGradient :: Test
+testMeanGradient = testCase "testMeanGradient" $ do
+    [dx] <- TF.runSession $ do
+        x <- TF.render $ TF.vector [1, 2, 0 :: Float]
+        let y = TF.mean x (TF.vector [0 :: Int32])
+        TF.gradients y [x] >>= TF.run
+    V.fromList [1, 1, 1 :: Float] @=? dx
+
+testMeanGradGrad :: Test
+testMeanGradGrad = testCase "testMeanGradGrad" $ do
+    [ddx] <- TF.runSession $ do
+        x <- TF.render $ TF.vector [1, 2, 0 :: Float]
+        let y = TF.mean x (TF.vector [0 :: Int32])
+        [dx] <- TF.gradients y [x]
+        TF.gradients dx [x] >>= TF.run
+
+    V.fromList [0, 0, 0 :: Float] @=? ddx
 
 testMaxGradient :: Test
 testMaxGradient = testCase "testMaxGradient" $ do
@@ -611,6 +628,8 @@ main = defaultMain
             , testCreateGraphNameScopes
             , testDiamond
             , testAddNGradient
+            , testMeanGradient
+            , testMeanGradGrad
             , testMaxGradient
             , testConcatGradient
             , testConcatGradientSimple