mark explicit (#4813)

Co-authored-by: Steve <ubuntu@ip-172-31-34-29.ap-northeast-1.compute.internal>

src/graph/transform/cuda/cuda_to_block.cu

@@ -43,7 +43,7 @@ namespace {
 template<typename IdType>
 class DeviceNodeMapMaker {
  public:
-  DeviceNodeMapMaker(
+  explicit DeviceNodeMapMaker(
       const std::vector<int64_t>& maxNodesPerType) :
       max_num_nodes_(0) {
     max_num_nodes_ = *std::max_element(maxNodesPerType.begin(),

src/rpc/network/msg_queue.h

@@ -93,7 +93,7 @@ class MessageQueue {
    * \param queue_size size (bytes) of message queue
    * \param num_producers number of producers, use 1 by default
    */
-  MessageQueue(int64_t queue_size /* in bytes */,
+  explicit MessageQueue(int64_t queue_size /* in bytes */,
                int num_producers = 1);
 
   /*!