Function ForwardFanins

tensorflow/core/grappler/optimizers/model_pruner.cc:170–212

ForwardFanins forwards fanins of a node to be removed to its fanouts. This currently is specific to nodes defined in `IsTrivialOp` under the assumption they can have at most one regular fanin (at index 0) and one regular fanout index (at 0). The forwarding is as follows: If the node to be removed has a regular fanin (at index 0), that fanin will be forwarded by replacing the fanin (consisting of

//   controlled fanout.
// TODO(lyandy): Move this to a shared util for GraphView.
Status ForwardFanins(utils::MutableGraphView* graph_view, int node_to_delete,
                     absl::flat_hash_set<int>* mutated_fanouts) {
  utils::Mutation* mutation = graph_view->GetMutationBuilder();
  auto* node_view = graph_view->GetNode(node_to_delete);
  std::vector<absl::string_view> controlling_fanin_names;
  controlling_fanin_names.reserve(node_view->NumControllingFanins());
  for (const auto& controlling_fanin : node_view->GetControllingFanins()) {
    controlling_fanin_names.push_back(controlling_fanin.node_view()->GetName());
  }
  const auto& node_regular_fanin_0 = node_view->GetRegularFanin(0);
  const bool has_regular_fanin_0 = node_view->NumRegularFanins() >= 1;
  const string regular_fanin_0_name =
      has_regular_fanin_0 ? node_regular_fanin_0.node_view()->GetName() : "";

  // Forward to regular fanouts.
  if (has_regular_fanin_0) {
    TensorId tensor_id(regular_fanin_0_name, node_regular_fanin_0.index());
    for (const auto& fanout : node_view->GetRegularFanout(0)) {
      auto* fanout_node_view = fanout.node_view();
      mutation->AddOrUpdateRegularFanin(fanout_node_view, fanout.index(),
                                        tensor_id);
      for (const auto& controlling_fanin : controlling_fanin_names) {
        mutation->AddControllingFanin(fanout_node_view, controlling_fanin);
      }
      mutated_fanouts->emplace(fanout.node_index());
    }
  }

  // Forward to controlled fanouts.
  for (const auto& controlled_fanout : node_view->GetControlledFanouts()) {
    auto* fanout_node_view = controlled_fanout.node_view();
    mutation->RemoveControllingFanin(fanout_node_view, node_view->GetName());
    if (has_regular_fanin_0) {
      mutation->AddControllingFanin(fanout_node_view, regular_fanin_0_name);
    }
    for (const auto& controlling_fanin : controlling_fanin_names) {
      mutation->AddControllingFanin(fanout_node_view, controlling_fanin);
    }
    mutated_fanouts->emplace(controlled_fanout.node_index());
  }

  return mutation->Apply();
}

absl::flat_hash_map<int, std::vector<bool>> IdentityNTerminalPorts(
    const utils::MutableGraphView& graph_view,