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

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

src/objective/hinge.cu

@@ -27,6 +27,8 @@ class HingeObj : public ObjFunction {
   void Configure(
       const std::vector<std::pair<std::string, std::string> > &args) override {}
 
+  ObjInfo Task() const override { return {ObjInfo::kRegression, false}; }
+
   void GetGradient(const HostDeviceVector<bst_float> &preds,
                    const MetaInfo &info,
                    int iter,