add some comments

Author: Yancey0623

paddle/fluid/framework/details/multi_devices_graph_pass.cc

@@ -431,7 +431,9 @@ std::unique_ptr<ir::Graph> MultiDevSSAGraphBuilder::ApplyImpl(
                     CreateReduceOp(&result, g_name, cur_device_id);
                     graph->Get<ShardedVarDevice>(kShardedVarDevice)
                         .emplace(g_name, cur_device_id);
-                    bcast_var_name_set[cur_device_id].emplace(p_name);
+                    if (!is_dist_train) {
+                      bcast_var_name_set[cur_device_id].emplace(p_name);
+                    }
                     break;
                   case BuildStrategy::ReduceStrategy::kAllReduce:
                     if (IsSparseGradient(g_name)) {
@@ -461,7 +463,7 @@ std::unique_ptr<ir::Graph> MultiDevSSAGraphBuilder::ApplyImpl(
   if ((use_gpu &&
        strategy_.reduce_ == BuildStrategy::ReduceStrategy::kReduce) ||
       is_dist_train) {
-    // Insert BCast Ops
+    // allways broadcast receieved parameters for distributed training
     for (size_t dev_id = 0; dev_id < bcast_var_name_set.size(); ++dev_id) {
       auto &to_bcast_set = bcast_var_name_set[dev_id];
       for (auto &bcast_name : to_bcast_set) {