Pass infomation about objective to tree methods. (#7385)

* Define the `ObjInfo` and pass it down to every tree updater.

src/objective/aft_obj.cu

@@ -38,6 +38,8 @@ class AFTObj : public ObjFunction {
     param_.UpdateAllowUnknown(args);
   }
 
+  ObjInfo Task() const override { return {ObjInfo::kSurvival, false}; }
+
   template <typename Distribution>
   void GetGradientImpl(const HostDeviceVector<bst_float> &preds,
                        const MetaInfo &info,