[GitHub] [incubator-mxnet] apeforest commented on a change in pull request #15253: Add higher order gradient support `sigmoid`, `tan`, `tanh`

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apeforest commented on a change in pull request #15253: Add higher order gradient support `sigmoid`, `tan`, `tanh`
URL: https://github.com/apache/incubator-mxnet/pull/15253#discussion_r294609295
 
 

 ##########
 File path: src/operator/tensor/elemwise_unary_op_basic.cc
 ##########
 @@ -121,7 +121,20 @@ The storage type of ``sigmoid`` output is always dense
 .set_attr<nnvm::FGradient>("FGradient", ElemwiseGradUseOut{"_backward_sigmoid"});
 
 MXNET_OPERATOR_REGISTER_BINARY_WITH_SPARSE_CPU(_backward_sigmoid,
-                                               unary_bwd<mshadow_op::sigmoid_grad>);
+                                               unary_bwd<mshadow_op::sigmoid_grad>)
+.set_attr<nnvm::FGradient>("FGradient",
+  [](const nnvm::NodePtr& n, const std::vector<nnvm::NodeEntry>& ograds) {
+    auto fx = nnvm::NodeEntry{n->inputs[1]};
+    auto gx_ograd = nnvm::NodeEntry{n};
+
+    std::vector<nnvm::NodeEntry> ret;
+       
+    ret.emplace_back(MakeNode("elemwise_mul", n->attrs.name + "_backward_grad_grad_inp",
 
 Review comment:
   Shouldn't the first item be _backward_grad_grad and the second item be _backward_grad_grad_inp?
   
   Given that:
   ```
   n->input[0]:  ygrad
   n->input[1]: y
   ```
   

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