[PASS] Make placedevice compatible with backward op (#88)

nnvm/src/pass/place_device.cc

@@ -25,6 +25,8 @@ Graph PlaceDevice(Graph src) {
   const Op* copy_op = Op::Get(src.GetAttr<std::string>("device_copy_op"));
   auto& device_assign_map = src.GetAttr<DeviceAssignMap>("device_assign_map");
   const IndexedGraph& idx = src.indexed_graph();
+  static auto& is_backward =
+      Op::GetAttr<TIsBackward>("TIsBackward");
   DeviceVector device;
   // copy on write semanatics
   if (src.attrs.count("device") != 0) {
@@ -45,9 +47,16 @@ Graph PlaceDevice(Graph src) {
           << "The device assignment not found for group " << device_group;
       device[nid] = dit->second;
     } else {
-      for (const IndexedGraph::NodeEntry& e : inode.inputs) {
-        if (device[e.node_id] != -1) {
-          device[nid] = device[e.node_id]; break;
+      if (!inode.source->is_variable() &&
+          is_backward.get(inode.source->op(), false)) {
+        if (device[inode.control_deps[0]] != -1) {
+          device[nid] = device[inode.control_deps[0]];
+        }
+      } else {
+        for (const IndexedGraph::NodeEntry& e : inode.inputs) {
+          if (device[e.node_id] != -1) {
+            device[nid] = device[e.node_id]; break;
+          }
         }
       }
     }