Fix sequence expand op (#11618)

* Set zero outside functor

paddle/fluid/operators/sequence_expand_op.h

@@ -151,9 +151,6 @@ struct SequenceExpandGradFunctor<platform::CPUDeviceContext, T> {
       const framework::Vector<size_t>& x_lod,   /*expand source lod*/
       const framework::Vector<size_t>& ref_lod, /*expand referenced lod*/
       LoDTensor* dx) {
-    math::SetConstant<platform::CPUDeviceContext, T> set_zero;
-    set_zero(context, dx, static_cast<T>(0));
-
     int dout_offset = 0;
     for (size_t i = 1; i < ref_lod.size(); ++i) {
       int repeat_num = ref_lod[i] - ref_lod[i - 1];
@@ -187,6 +184,10 @@ class SequenceExpandGradKernel : public framework::OpKernel<T> {
     g_x->mutable_data<T>(context.GetPlace());
     g_x->set_lod(x->lod());
 
+    auto& dev_ctx = context.template device_context<DeviceContext>();
+    math::SetConstant<DeviceContext, T> set_zero;
+    set_zero(dev_ctx, g_x, static_cast<T>(0));
+
     auto& y_lod = y->lod();
     if (ref_level == -1) ref_level = y_lod.size() - 1;
     // just copy the gradient