message: set_cpu needs dst to already have "the right structure", but test_dst was
  uninitialized. Now it's initialized from src.
diff: @@ -136,7 +136,7 @@ size_t conf_gpu::idx_cpu2gpu(size_t cpu_val_idx, size_t cpu_node_idx, const gpu_
  static
  bool constructor_valid(const conf_gpu& gpu, const conf& src, const gpu_data& d){
  - conf test_dst;
  + conf test_dst = src;
  gpu.set_cpu(test_dst, d);
  sz n = src.num_floats();
  assert(n == test_dst.num_floats());
  