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

[GitBox <gi...@apache.org>]

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_r272024151
 
 

 ##########
 File path: src/imperative/imperative.cc
 ##########
 @@ -347,8 +347,9 @@ std::vector<NDArray*> Imperative::Backward(
       x_reqs.push_back(info.grad_req);
       info.fresh_out_grad = true;
     }
-    CHECK_GT(xs.size(), 0)
-        << "There are no inputs in computation graph that require gradients.";
+    if (xs.empty()) {
 
 Review comment:
   Current backward operation requires an operator must have at least one inputs, because the gradient of a constants is always zero. However, the second order of some operators such as relu is actually gradient of a constant (ones or zeros). Therefore we need to support gradient for constant operators.

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