Fix.

Author: trivialfis

src/data/metainfo.h

@@ -38,7 +38,8 @@ inline MetaField MapMetaField(StringView key) {
     return MetaField::kLabelUpperBound;
   } else if (key == "feature_weights") {
     return MetaField::kFeatureWeights;
-  } else if (key == "group_ptr") {
+  } else if (key == "group_ptr" || key == "group") {
+    // `group_ptr` is output, `group` is input.
     return MetaField::kGroupPtr;
   } else if (key == "qid") {
     return MetaField::kQid;