message: add description of num_item
diff: @@ -627,6 +627,9 @@ int compute_items_per_thread(std::shared_ptr<const CudaExecutor> exec)
  {
  const int version = exec->get_major_version()
  << 4 + exec->get_minor_version();
  + // The num_item is decided to make the occupancy 100%
  + // TODO: Extend this list when new GPU is released
  + // Tune this parameter
  // 128 threads/block the number of items per threads
  // 3.0 3.5: 6
  // 3.7: 14
  @@ -648,7 +651,7 @@ int compute_items_per_thread(std::shared_ptr<const CudaExecutor> exec)
  case 0x37:
  num_item = 14;
  }
  - // Ensure that satisfy
  + // Ensure that satisfy:
  // sizeof(IndexType) + sizeof(ValueType)
  // <= items_per_thread * sizeof(IndexType)
  constexpr int minimal_num =
  