Fix ElemwiseSum for more than 4 inputs

src/operator/tensor/elemwise_sum.h

@@ -94,7 +94,7 @@ void ElementWiseSumCompute_(const nnvm::NodeAttrs& attrs,
       Kernel<Sum, xpu>::Launch(s, out_size, out_dptr, req[0], in_0_dptr);
       for (size_t i = 1; i < size; ++i) {
         DType* in_dptr = in_data[i].dptr<DType>();
-        Kernel<Sum, xpu>::Launch(s, out_size, out_dptr, req[0], out_dptr, in_dptr);
+        Kernel<Sum, xpu>::Launch(s, out_size, out_dptr, kWriteTo, out_dptr, in_dptr);
       }
       break;
     }

tests/python/unittest/test_operator.py

@@ -9875,6 +9875,25 @@ def test_grad(input_shape, kernel, stride=1, dilate=1, pad=0):
         pad         = 1
     )
 
+def test_elemwise_sum_for_gradient_accumulation():
+    for nrepeat in range(1, 10):
+        stored_grad = dict()
+        for grad_req in ['write', 'add']:
+            a = mx.nd.array([1])
+            b = mx.nd.array([2])
+            if grad_req == 'write':
+                a.attach_grad(grad_req='write')
+            elif grad_req == 'add':
+                a.attach_grad(grad_req='add')
+            a.grad[:] = 0
+            with mx.autograd.record():
+                for _ in range(nrepeat):
+                    b = b * a
+                b.backward()
+            stored_grad[grad_req] = a.grad.asscalar()
+        assert stored_grad['write'] == stored_grad['add']
+        assert stored_grad['write'] == 2 * nrepeat
+
 
 if __name__ == '__main__':
     import nose