Softmax grad op (#3164)

* init softmax grad op

* add compute code

* export Backward to python

* update test ,export op.type to python

* update python test, fix compute bug

* update unit test

* use eigen

* optimize eigen code

* add gpu test

* register softmax_grad GPU kernel and fix test bug

* typo

* follow comments

paddle/framework/operator.h

@@ -55,6 +55,10 @@ class OperatorBase {
   /// e.g. Variable "x@GRAD" is the gradient of varibale "x".
   static std::string GRAD_VAR_SUFFIX() { return "@GRAD"; }
 
+  static std::string GRAD_VAR_NAME(const std::string& name) {
+    return name + GRAD_VAR_SUFFIX();
+  }
+
   /// Variables with this suffix are supposed to be filled up with zeros.
   static std::string ZERO_VAR_SUFFIX() { return "@ZERO"; }
 

paddle/operators/softmax_op.cc

 /* Copyright (c) 2016 PaddlePaddle Authors. All Rights Reserve.
 
-   Licensed under the Apache License, Version 2.0 (the "License");
-   you may not use this file except in compliance with the License.
-   You may obtain a copy of the License at
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
 
     http://www.apache.org/licenses/LICENSE-2.0
 
-   Unless required by applicable law or agreed to in writing, software
-   distributed under the License is distributed on an "AS IS" BASIS,
-   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-   See the License for the specific language governing permissions and
-   limitations under the License. */
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License. */
+
 #include "paddle/operators/softmax_op.h"
 
 namespace paddle {
@@ -19,12 +20,13 @@ namespace operators {
 class SoftmaxOp : public OperatorWithKernel {
 protected:
   void InferShape(const InferShapeContext &ctx) const override {
-    PADDLE_ENFORCE(ctx.InputSize() == 1, "Only one input is need for softmax");
-    PADDLE_ENFORCE(ctx.Input<Tensor>(0)->dims().size() == 2,
+    PADDLE_ENFORCE(ctx.InputSize() == 1UL,
+                   "Only one input is need for softmax");
+    PADDLE_ENFORCE(ctx.Input<Tensor>("X")->dims().size() == 2UL,
                    "The input of softmax op must be matrix");
-    PADDLE_ENFORCE(ctx.OutputSize() == 1,
+    PADDLE_ENFORCE(ctx.OutputSize() == 1UL,
                    "Only one output is need for softmax");
-    ctx.Output<Tensor>(0)->Resize(ctx.Input<Tensor>(0)->dims());
+    ctx.Output<Tensor>("Y")->Resize(ctx.Input<Tensor>("X")->dims());
   }
 };
 
@@ -40,10 +42,19 @@ public:
 
 class SoftmaxOpGrad : public OperatorWithKernel {
 protected:
-  void InferShape(const InferShapeContext &ctx) const override {}
-  std::string DebugString() const override {
-    LOG(INFO) << "SoftmaxOpGrad";
-    return "";
+  void InferShape(const InferShapeContext &ctx) const override {
+    PADDLE_ENFORCE(ctx.InputSize() == 3UL,
+                   "Input of SoftmaxOpGrad should be 3, X, Y, YG");
+    PADDLE_ENFORCE(ctx.OutputSize() == 1UL,
+                   "Output of SoftmaxOpGrad should be 1");
+    PADDLE_ENFORCE(ctx.InputVar("Y") != nullptr, "Input(Y) should not be null");
+    PADDLE_ENFORCE(ctx.InputVar(GRAD_VAR_NAME("Y")) != nullptr,
+                   "Input(Y@GRAD) should not be null");
+    PADDLE_ENFORCE(ctx.Input<Tensor>("Y")->dims() ==
+                       ctx.Input<Tensor>(GRAD_VAR_NAME("Y"))->dims(),
+                   "the shape of Input(0) and Input(1) should be the same");
+    ctx.Output<Tensor>(GRAD_VAR_NAME("X"))
+        ->Resize(ctx.Input<Tensor>("Y")->dims());
   }
 };
 
@@ -51,5 +62,7 @@ protected:
 }  // namespace paddle
 
 REGISTER_OP(softmax, ops::SoftmaxOp, ops::SoftmaxOpMaker);
-REGISTER_GRADIENT_OP(softmax, softmax_grad, ops::SoftmaxOpGrad);
 REGISTER_OP_CPU_KERNEL(softmax, ops::SoftmaxKernel<ops::CPUPlace, float>);
+REGISTER_GRADIENT_OP(softmax, softmax_grad, ops::SoftmaxOpGrad);
+REGISTER_OP_CPU_KERNEL(softmax_grad,
+                       ops::SoftmaxGradKernel<ops::CPUPlace, float>);

paddle/operators/softmax_op.cu

@@ -3,3 +3,4 @@
 #include "paddle/operators/softmax_op.h"
 
 REGISTER_OP_GPU_KERNEL(softmax, ops::SoftmaxKernel<ops::GPUPlace, float>);
+REGISTER_OP_GPU_KERNEL(softmax_grad, ops::SoftmaxGradKernel<ops::GPUPlace, float>);

paddle/operators/softmax_op.h

 /* Copyright (c) 2016 PaddlePaddle Authors. All Rights Reserve.
 
-   Licensed under the Apache License, Version 2.0 (the "License");
-   you may not use this file except in compliance with the License.
-   You may obtain a copy of the License at
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
 
     http://www.apache.org/licenses/LICENSE-2.0
 
-   Unless required by applicable law or agreed to in writing, software
-   distributed under the License is distributed on an "AS IS" BASIS,
-   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-   See the License for the specific language governing permissions and
-   limitations under the License. */
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License. */
 
 #pragma once
 
+#include "paddle/framework/ddim.h"
+#include "paddle/framework/operator.h"
+#include "paddle/framework/tensor.h"
 #include "paddle/operators/type_alias.h"
 
 namespace paddle {
@@ -23,8 +26,8 @@ template <typename Place, typename T>
 class SoftmaxKernel : public OpKernel {
 public:
   void Compute(const ExecutionContext& context) const override {
-    auto input = context.Input<Tensor>(0);
-    auto output = context.Output<Tensor>(0);
+    auto input = context.Input<Tensor>("X");
+    auto output = context.Output<Tensor>("Y");
     output->mutable_data<T>(context.GetPlace());
 
     auto logits = EigenMatrix<T>::From(*input);
@@ -57,5 +60,38 @@ public:
              .broadcast(one_by_class));
   }
 };
+
+template <typename Place, typename T>
+class SoftmaxGradKernel : public OpKernel {
+public:
+  void Compute(const ExecutionContext& context) const override {
+    std::shared_ptr<Tensor> scale_ = std::make_shared<Tensor>();
+
+    auto Y = context.Input<Tensor>("Y");
+    auto dY = context.Input<Tensor>(OperatorBase::GRAD_VAR_NAME("Y"));
+    auto dX = context.Output<Tensor>(OperatorBase::GRAD_VAR_NAME("X"));
+    dX->mutable_data<T>(context.GetPlace());
+
+    const int batch_size = Y->dims()[0];
+    const int class_num = Y->dims()[1];
+
+    Eigen::DSizes<int, 1> along_class(1);
+    Eigen::DSizes<int, 2> batch_by_one(batch_size, 1);
+    Eigen::DSizes<int, 2> one_by_class(1, class_num);
+
+    auto Y_eigen = EigenMatrix<T>::From(*Y);
+    auto dY_eigen = EigenMatrix<T>::From(*dY);
+    auto dX_eigen = EigenMatrix<T>::From(*dX);
+    auto place = context.GetEigenDevice<Place>();
+
+    auto dot = (Y_eigen * dY_eigen)
+                   .sum(along_class)
+                   .eval()
+                   .reshape(batch_by_one)
+                   .broadcast(one_by_class);
+    dX_eigen.device(place) = (dY_eigen - dot) * Y_eigen;
+  }
+};
+
 }  // namespace operators
 }  // namespace paddle

paddle/operators/type_alias.h

@@ -22,6 +22,7 @@ namespace paddle {
 namespace operators {
 
 using OpKernel = framework::OpKernel;
+using OperatorBase = framework::OperatorBase;
 using InferShapeContext = framework::InferShapeContext;
 using ExecutionContext = framework::ExecutionContext;
 using Variable = framework::Variable;

python/paddle/v2/framework/tests/test_softmax_op.py

 import unittest
-from op_test_util import OpTestMeta
+
 import numpy as np
+import paddle.v2.framework.core as core
+import paddle.v2.framework.create_op_creation_methods as creation
+
+from op_test_util import OpTestMeta
 
 
 def stable_softmax(x):
@@ -19,5 +23,63 @@ class TestSoftmaxOp(unittest.TestCase):
         self.Y = np.apply_along_axis(stable_softmax, 1, self.X)
 
 
+class TestSoftmaxGradOp(unittest.TestCase):
+    def test_softmax_grad(self):
+        op = creation.op_creations.softmax(X="X", Y="Y")
+        backward_op = core.Operator.backward(op, set())
+        self.assertEqual(backward_op.type(), "softmax_grad")
+        expected = '''Op(softmax_grad), inputs:(X, Y, Y@GRAD), outputs:(X@GRAD).'''
+        self.assertEqual(expected, str(backward_op))
+
+        batch_size = 3
+        class_num = 5
+        # Initialize X and add 1e-2 for numerical stability
+        Y = np.random.rand(batch_size, class_num).astype(np.float32)
+        Y = Y + 1e-2
+        dY = np.random.rand(batch_size, class_num).astype(np.float32)
+
+        # Reference implementation of cross entropy with soft labels
+        def label_softmax_grad(Y, dY):
+            dX = Y * 0.0
+            for i in range(batch_size):
+                d = np.dot(Y[i, :], dY[i, :])
+                dX[i, :] = Y[i, :] * (dY[i, :] - d)
+            return dX
+
+        expected = label_softmax_grad(Y, dY)
+
+        scope = core.Scope()
+        places = []
+        places.append(core.CPUPlace())
+        if core.is_compile_gpu():
+            places.append(core.GPUPlace(0))
+
+        for place in places:
+            y = scope.new_var("Y")
+            y_tensor = y.get_tensor()
+            y_tensor.set_dims([batch_size, class_num])
+            y_tensor.alloc_float(place)
+            y_tensor.set(Y, place)
+
+            dy = scope.new_var("Y@GRAD")
+            dy_tensor = dy.get_tensor()
+            dy_tensor.set_dims([batch_size, class_num])
+            dy_tensor.alloc_float(place)
+            dy_tensor.set(dY, place)
+
+            x = scope.new_var("X")
+            dx = scope.new_var("X@GRAD")
+
+            tensor = scope.find_var("X@GRAD").get_tensor()
+            backward_op.infer_shape(scope)
+            self.assertEqual([batch_size, class_num], tensor.shape())
+
+            ctx = core.DeviceContext.create(place)
+            backward_op.run(scope, ctx)
+            actual = np.array(tensor)
+
+            np.testing.assert_almost_equal(actual, expected, decimal=3)
+
+
 if __name__ == '__main__':
     unittest.main()