[GitHub] [incubator-mxnet] apeforest commented on a change in pull request #14613: [MXNET-978] Higher order gradient support for some unary operators

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apeforest commented on a change in pull request #14613: [MXNET-978] Higher order gradient support for some unary operators
URL: https://github.com/apache/incubator-mxnet/pull/14613#discussion_r290913893
 
 

 ##########
 File path: src/operator/tensor/elemwise_unary_op_basic.cc
 ##########
 @@ -85,8 +85,23 @@ The storage type of ``relu`` output depends upon the input storage type:
 )code" ADD_FILELINE)
 .set_attr<nnvm::FGradient>("FGradient", ElemwiseGradUseOut{"_backward_relu"});
 
-MXNET_OPERATOR_REGISTER_BINARY_WITH_SPARSE_CPU(_backward_relu,
-                                               unary_bwd<mshadow_op::relu_grad>);
+MXNET_OPERATOR_REGISTER_BINARY_WITH_SPARSE_CPU(_backward_relu, unary_bwd<mshadow_op::relu_grad>)
+.set_attr<nnvm::FGradient>("FGradient",
+    [](const nnvm::NodePtr& n, const std::vector<nnvm::NodeEntry>& ograds) {
+      std::vector<nnvm::NodeEntry> ret;
+      // ograds[0]: d^2L/dx^2
+      // inputs[0]: dL/dy
+      // inputs[1]: y
+      // f(x) -> relu(x)
+      // f'(x) = 1 if x > 0 else 0
+      // f''(x) = 0
+      auto gx = nnvm::NodeEntry{n};  // f'(x)
+      ret.emplace_back(MakeNode("elemwise_mul", n->attrs.name + "_backward_grad_grad",
+                                {ograds[0], gx}, nullptr, &n));
 
 Review comment:
   Updated

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