Add mtx to ncclAllReduce

paddle/fluid/framework/parallel_executor.cc

@@ -340,6 +340,8 @@ ncclDataType_t ToNCCLDataType(std::type_index type) {
   }
 }
 
+static std::mutex g_nccl_mtx_;
+
 struct NCCLAllReduceOpHandle : public OpHandle {
   ParallelExecutorPrivate *member_;
 
@@ -361,6 +363,8 @@ struct NCCLAllReduceOpHandle : public OpHandle {
       int dtype = -1;
       size_t numel = 0;
 
+      std::lock_guard<std::mutex> g(g_nccl_mtx_);
+
       platform::dynload::ncclGroupStart();
 
       for (size_t i = 0; i < member_->local_scopes_.size(); ++i) {