nix: support Python tokenizer conversion in the router (#2515)

Ideally we wouldn't have the router wrapper that this change adds,
but when I give PyO3 a Python interpreter with packages, it ends
up linking libpython from the Python interpreter rather than the
constructed environment and cannot pick up the Python modules as
a result.

flake.nix

@@ -46,9 +46,26 @@
         launcher = cargoNix.workspaceMembers.text-generation-launcher.build.override {
           inherit crateOverrides;
         };
-        router = cargoNix.workspaceMembers.text-generation-router-v3.build.override {
+        router =
+          let
+            routerUnwrapped = cargoNix.workspaceMembers.text-generation-router-v3.build.override {
               inherit crateOverrides;
             };
+            packagePath =
+              with pkgs.python3.pkgs;
+              makePythonPath [
+                protobuf
+                sentencepiece
+                torch
+                transformers
+              ];
+          in
+          pkgs.writeShellApplication {
+            name = "text-generation-router";
+            text = ''
+              PYTHONPATH="${packagePath}" ${routerUnwrapped}/bin/text-generation-router "$@"
+            '';
+          };
         server = pkgs.python3.pkgs.callPackage ./nix/server.nix { inherit nix-filter; };
       in
       {

nix/crate-overrides.nix

@@ -28,6 +28,9 @@ defaultCrateOverrides
       ];
     };
   };
+  pyo3-build-config = attrs: {
+    buildInputs = [ python3 ];
+  };
   text-generation-benchmark = attrs: {
     src = filter {
       root = ../benchmark;