message: Refactor code to improve TensorDataSet construction speed.
diff: @@ -44,12 +44,15 @@ class TensorDataSet {
  int column_count = 0;
  for (int i = 0; i < input_spec_.dense_size(); ++i) {
  for (int j = 0; j < input_spec_.dense(i).size(); ++j) {
  - decision_trees::FeatureId id;
  - id.mutable_id()->set_value(strings::StrCat(column_count));
  - available_features_.push_back(id);
  ++column_count;
  }
  }
  + available_features_.reserve(column_count);
  + decision_trees::FeatureId id;
  + for (int i = 0; i < column_count; i++) {
  + id.mutable_id()->set_value(strings::StrCat(i));
  + available_features_.emplace_back(id);
  + }
  // Set up the random number generator.
  if (split_sampling_random_seed_ == 0) {
  