message: Be more careful in IsInitalized, and log when it is called on an unknown
  node_id.
diff: @@ -96,7 +96,12 @@ void SplitCollectionOperator::AddExample(
  }
  bool SplitCollectionOperator::IsInitialized(int32 node_id) const {
  - return stats_.at(node_id)->IsInitialized();
  + auto it = stats_.find(node_id);
  + if (it == stats_.end()) {
  + LOG(WARNING) << "IsInitialized called with unknown node_id = " << node_id;
  + return false;
  + }
  + return it->second->IsInitialized();
  }
  void SplitCollectionOperator::CreateAndInitializeCandidateWithExample(
  