Integer gradient summation for GPU histogram algorithm. (#2681)

src/objective/rank_obj.cc

@@ -109,10 +109,8 @@ class LambdaRankObj : public ObjFunction {
           bst_float g = p - 1.0f;
           bst_float h = std::max(p * (1.0f - p), eps);
           // accumulate gradient and hessian in both pid, and nid
-          gpair[pos.rindex].grad += g * w;
-          gpair[pos.rindex].hess += 2.0f * w * h;
-          gpair[neg.rindex].grad -= g * w;
-          gpair[neg.rindex].hess += 2.0f * w * h;
+          gpair[pos.rindex] += bst_gpair(g * w, 2.0f*w*h);
+          gpair[neg.rindex] += bst_gpair(-g * w, 2.0f*w*h);
         }
       }
     }