python3Packages.torch: fix cudnn attribute access

Without the change the eval fails as:

    $ nix build --no-link -f. opensplatWithCuda --argstr system aarch64-linux
    error:
       error: attribute 'lib' missing
       at /home/slyfox/dev/git/nixpkgs-master/pkgs/development/python-modules/torch/default.nix:357:30:
          356|       export CUDNN_INCLUDE_DIR=${lib.getLib cudnn}/include
          357|       export CUDNN_LIB_DIR=${cudnn.lib}/lib
             |                              ^
          358|     ''

Author: trofi

pkgs/development/python-modules/torch/source/default.nix

@@ -364,7 +364,7 @@ buildPythonPackage rec {
     ''
     + lib.optionalString (cudaSupport && cudaPackages ? cudnn) ''
       export CUDNN_INCLUDE_DIR=${lib.getLib cudnn}/include
-      export CUDNN_LIB_DIR=${cudnn.lib}/lib
+      export CUDNN_LIB_DIR=${lib.getLib cudnn}/lib
     ''
     + lib.optionalString rocmSupport ''
       export ROCM_PATH=${rocmtoolkit_joined}