caffe: use pre-CUDNN 8.x

pkgs/applications/science/math/caffe/default.nix

@@ -21,7 +21,13 @@
 }:
 
 let
-  inherit (cudaPackages) cudatoolkit cudnn nccl;
+  inherit (cudaPackages) cudatoolkit nccl;
+  # The default for cudatoolkit 10.1 is CUDNN 8.0.5, the last version to support CUDA 10.1.
+  # However, this caffe does not build with CUDNN 8.x, so we use CUDNN 7.6.5 instead.
+  # Earlier versions of cudatoolkit use pre-8.x CUDNN, so we use the default.
+  cudnn = if lib.versionOlder cudatoolkit.version "10.1"
+    then cudaPackages.cudnn
+    else cudaPackages.cudnn_7_6_5;
 in
 
 assert leveldbSupport -> (leveldb != null && snappy != null);