message: Fetch back tensors from recently release engine. Fixes a couple of unit
  tests.
diff: @@ -1056,6 +1056,7 @@ void PoplarExecutor::AboutToFreeEngine(poplar::Engine* engine) {
  if (current_engine_ != nullptr) {
  std::lock_guard<std::recursive_mutex> g(mutex_);
  if (engine == current_engine_) {
  + MoveDeviceToHost();
  DeferredDeallocation();
  current_engine_ = NULL;
  }
  