fix topk softmax performance issue (#14702)

sgl-kernel/csrc/cpu/topk.cpp

@@ -227,11 +227,9 @@ void topk_softmax_kernel_impl(
         queue[e] = {scores[e], e};
       }
 
-      std::partial_sort(
-          queue.begin(),
-          queue.begin() + num_experts_per_group,
-          queue.end(),
-          [](const elem_t& x, const elem_t& y) -> bool { return x.first > y.first; });
+      std::partial_sort(queue.begin(), queue.begin() + topk, queue.end(), [](const elem_t& x, const elem_t& y) -> bool {
+        return x.first > y.first;
+      });
 
       for (int64_t j = 0; j < topk; ++j) {
         topk_weights[i * topk + j] = queue[j].first;