Add impureWithCuda dev shell

This shell is handy when developing some kernels jointly with TGI - it
adds nvcc and a bunch of commonly-used CUDA libraries to the environment.

We don't add this to the normal impure shell to keep the development
environment as clean as possible (avoid accidental dependencies, etc.).

flake.nix

@@ -137,6 +137,11 @@
 
           impure = callPackage ./nix/impure-shell.nix { inherit server; };
 
+          impureWithCuda = callPackage ./nix/impure-shell.nix {
+            inherit server;
+            withCuda = true;
+          };
+
           impure-flash-attn-v1 = callPackage ./nix/impure-shell.nix {
             server = server.override { flash-attn = python3.pkgs.flash-attn-v1; };
           };

nix/impure-shell.nix

@@ -1,7 +1,12 @@
 {
+  lib,
   mkShell,
   black,
+  cmake,
   isort,
+  ninja,
+  which,
+  cudaPackages,
   openssl,
   pkg-config,
   protobuf,
@@ -11,14 +16,17 @@
   ruff,
   rust-bin,
   server,
+
+  # Enable dependencies for building CUDA packages. Useful for e.g.
+  # developing marlin/moe-kernels in-place.
+  withCuda ? false,
 }:
 
 mkShell {
-  buildInputs =
+  nativeBuildInputs =
     [
       black
       isort
-      openssl.dev
       pkg-config
       (rust-bin.stable.latest.default.override {
         extensions = [
@@ -31,6 +39,19 @@ mkShell {
       redocly
       ruff
     ]
+    ++ (lib.optionals withCuda [
+      cmake
+      ninja
+      which
+
+      # For most Torch-based extensions, setting CUDA_HOME is enough, but
+      # some custom CMake builds (e.g. vLLM) also need to have nvcc in PATH.
+      cudaPackages.cuda_nvcc
+    ]);
+  buildInputs =
+    [
+      openssl.dev
+    ]
     ++ (with python3.pkgs; [
       venvShellHook
       docker
@@ -40,10 +61,26 @@ mkShell {
       pytest
       pytest-asyncio
       syrupy
-    ]);
+    ])
+    ++ (lib.optionals withCuda (
+      with cudaPackages;
+      [
+        cuda_cccl
+        cuda_cudart
+        cuda_nvtx
+        libcublas
+        libcusolver
+        libcusparse
+      ]
+    ));
 
   inputsFrom = [ server ];
 
+  env = lib.optionalAttrs withCuda {
+    CUDA_HOME = "${lib.getDev cudaPackages.cuda_nvcc}";
+    TORCH_CUDA_ARCH_LIST = lib.concatStringsSep ";" python3.pkgs.torch.cudaCapabilities;
+  };
+
   venvDir = "./.venv";
 
   postVenvCreation = ''
@@ -51,6 +88,7 @@ mkShell {
     ( cd server ; python -m pip install --no-dependencies -e . )
     ( cd clients/python ; python -m pip install --no-dependencies -e . )
   '';
+
   postShellHook = ''
     unset SOURCE_DATE_EPOCH
     export PATH=$PATH:~/.cargo/bin