message: Add const to helper methods in grappler::NodeCosts.
diff: @@ -51,21 +51,21 @@ struct NodeCosts {
  int64 internal_write_bytes = 0;
  // Convenience functions.
  - int64 num_total_input_bytes() {
  + int64 num_total_input_bytes() const {
  return std::accumulate(num_input_bytes_accessed.begin(),
  num_input_bytes_accessed.end(), 0LL);
  }
  - int64 num_total_read_bytes() {
  + int64 num_total_read_bytes() const {
  return num_total_input_bytes() + internal_read_bytes;
  }
  - int64 num_total_output_bytes() {
  + int64 num_total_output_bytes() const {
  return std::accumulate(num_output_bytes_accessed.begin(),
  num_output_bytes_accessed.end(), 0LL);
  }
  - int64 num_total_write_bytes() {
  + int64 num_total_write_bytes() const {
  return num_total_output_bytes() + internal_write_bytes;
  }
  - int64 num_bytes_accessed() {
  + int64 num_bytes_accessed() const {
  return num_total_read_bytes() + num_total_write_bytes();
  }
  