bench: make exo-bench available via nix run on all platforms (#1415)

exo-bench was gated behind isDarwin in python/parts.nix because it used
exoVenv, which pulls in MLX (Darwin-only). However, exo_bench.py is an
HTTP client that only needs loguru, transformers, huggingface-hub, and
tiktoken.

Made bench a uv workspace member with its own pyproject.toml declaring
only the minimal dependencies. Added a separate benchVenv in parts.nix
built from that workspace member, and moved exo-bench out of the
isDarwin block so it is available on all platforms.

Test plan:
- `nix run .#exo-bench -- --help` prints argparse help

---------

Co-authored-by: Ryuichi Leo Takashige <leo@exolabs.net>
Co-authored-by: rltakashige <rl.takashige@gmail.com>

python/parts.nix

@@ -69,6 +69,16 @@
         text = ''exec python ${path} "$@"'';
       };
 
+      benchVenv = pythonSet.mkVirtualEnv "exo-bench-env" {
+        exo-bench = [ ];
+      };
+
+      mkBenchScript = name: path: pkgs.writeShellApplication {
+        inherit name;
+        runtimeInputs = [ benchVenv ];
+        text = ''exec python ${path} "$@"'';
+      };
+
       mkSimplePythonScript = name: path: pkgs.writeShellApplication {
         inherit name;
         runtimeInputs = [ pkgs.python313 ];
@@ -96,8 +106,8 @@
           exo = exoPackage;
           # Test environment for running pytest outside of Nix sandbox (needs GPU access)
           exo-test-env = testVenv;
-          exo-bench = mkPythonScript "exo-bench" (inputs.self + /bench/exo_bench.py);
         } // {
+        exo-bench = mkBenchScript "exo-bench" (inputs.self + /bench/exo_bench.py);
         exo-get-all-models-on-cluster = mkSimplePythonScript "exo-get-all-models-on-cluster" (inputs.self + /tests/get_all_models_on_cluster.py);
       };