Writing/reading using Output/InputArchive

I am trying to implement serialisation functionality into my libtorch code. As my top-level class comprises quite many different member attributes I have implemented two user-callable functions

    inline void save(const std::string& filename,
                     const std::string& key="") const
      torch::serialize::OutputArchive archive;
      write(archive, key).save_to(filename);

    inline void load(const std::string& filename,
                     const std::string& key="")
      torch::serialize::InputArchive archive;
      read(archive, key);

Furthermore, I have implemented write and read functions of the form

    inline torch::serialize::OutputArchive& write(torch::serialize::OutputArchive& archive,
                                                  const std::string& key="") const
      archive.write(key+"dim", torch::full({1}, dim_));

      geo_.write(archive, key+"geo");
      rhs_.write(archive, key+"rhs");
      sol_.write(archive, key+"sol");

      torch::serialize::OutputArchive archive_net;

      archive.write(key+"net", archive_net);

      torch::serialize::OutputArchive archive_opt;

      archive.write(key+"opt", archive_opt);

      return archive;

inline torch::serialize::InputArchive& read(torch::serialize::InputArchive& archive,
                                                const std::string& key="")
      torch::Tensor tensor;

      archive.read(key+"dim", tensor);
      if (tensor.item<int64_t>() != dim_)
        throw std::runtime_error("dim mismatch");

      geo_.read(archive, key+"geo");
      rhs_.read(archive, key+"rhs");
      sol_.read(archive, key+"sol");

      torch::serialize::InputArchive archive_net;
      archive.read(key+"net", archive_net);

      torch::serialize::InputArchive archive_opt;
      archive.read(key+"opt", archive_opt);      

      return archive;

Here, net_ is of type MyGenerator using the generator approach described here.

I have the following problems:

  1. The code runs up to the point that opt_ is loaded, which gives rise to the exception
    libc++abi: terminating with uncaught exception of type c10::Error: loaded state dict contains a parameter group that has a different size than the optimizer's parameter group
    Exception raised from serialize at /tmp/libtorch-20220312-61658-dj98dd/torch/csrc/api/include/torch/optim/serialize.h:170 (most recent call first):
    frame #0: c10::detail::torchCheckFail(char const*, char const*, unsigned int, char const*) + 92 (0x10040d4c0 in libc10.dylib)
    frame #1: void torch::optim::serialize<torch::optim::AdamParamState, torch::optim::AdamOptions>(torch::serialize::InputArchive&, torch::optim::Optimizer&) + 832 (0x108880be8 in libtorch_cpu.dylib)
    frame #2: torch::optim::Adam::load(torch::serialize::InputArchive&) + 108 (0x10887f21c in libtorch_cpu.dylib)
  2. The net_ object is still empty after net_->load(archive_net).

Any help on any of the above issues is highly appreciated.

Kind regards,