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