torch: passthru.cudaCapabilities is available only when cudaSupport is true

  requiredSystemFeatures = [ "big-parallel" ];

  passthru = {

    inherit cudaSupport cudaPackages gpuTargetString;

    cudaCapabilities = supportedCudaCapabilities;

    inherit cudaSupport cudaPackages;

    # At least for 1.10.2 `torch.fft` is unavailable unless BLAS provider is MKL. This attribute allows for easy detection of its availability.

    blasProvider = blas.provider;

  } // lib.optionalAttrs cudaSupport {

    # NOTE: supportedCudaCapabilities isn't computed unless cudaSupport is true, so we can't use

    #   it in the passthru set above because a downstream package might try to access it even

    #   when cudaSupport is false. Better to have it missing than null or an empty list by default.

    cudaCapabilities = supportedCudaCapabilities;

  };

  meta = with lib; {

}:

let

  inherit (torch) cudaPackages gpuTargetString;

  inherit (torch) cudaPackages cudaCapabilities;

  inherit (cudaPackages) cudatoolkit cudaFlags cudaVersion;

  # NOTE: torchvision doesn't use cudnn; torch does!

  + lib.optionalString cudaSupport ''

    export CC=${cudatoolkit.cc}/bin/cc

    export CXX=${cudatoolkit.cc}/bin/c++

    export TORCH_CUDA_ARCH_LIST="${gpuTargetString}"

    export TORCH_CUDA_ARCH_LIST="${lib.concatStringsSep ";" cudaCapabilities}"

    export FORCE_CUDA=1

  '';