message: Update VirtualPlacer call to match new api
diff: @@ -904,7 +904,7 @@ class AutoMixedPrecisionImpl {
  AutoMixedPrecisionImpl(Cluster* cluster,
  const std::unordered_set<string>& nodes_to_preserve,
  GraphDef* graph, string id)
  - : virtual_placer_(cluster),
  + : virtual_placer_(cluster->GetDevices()),
  nodes_to_preserve_(nodes_to_preserve),
  graph_(graph),
  id_(id),
  