cudaPackages: overhaul of how we package cuda packages

There are many different versions of the `cudatoolkit` and related
cuda packages, and it can be tricky to ensure they remain compatible.

- `cudaPackages` is now a package set with `cudatoolkit`, `cudnn`, `cutensor`, `nccl`, as well as `cudatoolkit` split into smaller packages ("redist");
- expressions should now use `cudaPackages` as parameter instead of the individual cuda packages;
- `makeScope` is now used, so it is possible to use `.overrideScope'` to set e.g. a different `cudnn` version;
- `release-cuda.nix` is introduced to easily evaluate cuda packages using hydra.

doc/languages-frameworks/cuda.section.md

@@ -0,0 +1,34 @@
+# CUDA {#cuda}
+
+CUDA-only packages are stored in the `cudaPackages` packages set. This set
+includes the `cudatoolkit`, portions of the toolkit in separate derivations,
+`cudnn`, `cutensor` and `nccl`.
+
+A package set is available for each CUDA version, so for example
+`cudaPackages_11_6`. Within each set is a matching version of the above listed
+packages. Additionally, other versions of the packages that are packaged and
+compatible are available as well. For example, there can be a
+`cudaPackages.cudnn_8_3_2` package.
+
+To use one or more CUDA packages in an expression, give the expression a `cudaPackages` parameter, and in case CUDA is optional
+```nix
+cudaSupport ? false
+cudaPackages ? {}
+```
+
+When using `callPackage`, you can choose to pass in a different variant, e.g.
+when a different version of the toolkit suffices
+```nix
+mypkg = callPackage { cudaPackages = cudaPackages_11_5; }
+```
+
+If another version of say `cudnn` or `cutensor` is needed, you can override the
+package set to make it the default. This guarantees you get a consistent package
+set.
+```nix
+mypkg = let
+  cudaPackages = cudaPackages_11_5.overrideScope' (final: prev {
+    cudnn = prev.cudnn_8_3_2;
+  }});
+in callPackage { inherit cudaPackages; };
+```