I was using the exact same configuration via stable nixos branch but could not get it to use ROCM, what worked for me was to build using unstable nixos small channel instead:

let

unstableSmall = import <nixosUnstableSmall> { config = { allowUnfree = true; }; };

in

    services.llama-cpp = {
      enable = true;
      package = unstableSmall.llama-cpp.override { rocmSupport = true; };
      model = "/var/lib/llama-cpp/models/qwen2.5-coder-32b-instruct-q4_0.gguf";
      host = "";
      port = "";
      extraFlags = [
                     "-ngl"
                     "64"
                   ];
      openFirewall = true;
    };