message: Add feature dimension id and default direction in split node info.
diff: @@ -52,6 +52,16 @@ message BucketizedSplit {
  // the rule feature <= threshold.
  int32 feature_id = 1;
  int32 threshold = 2;
  + // If feature column is multivalent, this holds the index of the dimension
  + // for the split. Defaults to 0.
  + int32 dimension_id = 5;
  + enum DefaultDirection {
  + // Left is the default direction.
  + DEFAULT_LEFT = 0;
  + DEFAULT_RIGHT = 1;
  + }
  + // default direction for missing values.
  + DefaultDirection default_direction = 6;
  // Node children indexing into a contiguous
  // vector of nodes starting from the root.
  