b2.opts().WithControlInput(e).WithAttr(
                         kXlaHasHostTransferAttrName, true));

    Node* s = Sequencer(
        b2.opts().WithName("F1_sequencer").WithControlInputs({recv, send}),
        "F1");

    NodeBuilder node_builder("F1", "F1", lib_def.get());
    node_builder.Input(a).Input(b);
    Node* call =

  if (sequencer_ == nullptr) {
    NodeDef seq_def;
    // TODO(shikharagarwal): What source node should we use for errors?
    NodeDefBuilder builder(absl::StrCat(subgraph_name, "_sequencer"), "NoOp");
    builder.Attr(kXlaHostTransferSequencerAttr, subgraph_name);
    builder.Device(device_);
    Status s = builder.Finalize(&seq_def);
    if (!s.ok()) return s;

    FunctionLibraryDefinition* fld, std::unique_ptr<Graph>* host_graph) {
  host_graph->reset(new Graph(fld));

  // Create sequencer node in host graph.
  NodeDefBuilder sequencer_builder(absl::StrCat(xla_cluster_name, "_sequencer"),
                                   "NoOp");
  sequencer_builder.Attr("_xla_host_transfer_sequencer", xla_cluster_name);
  NodeDef sequencer_def;
  TF_RETURN_IF_ERROR(sequencer_builder.Finalize(&sequencer_def));