vllm: 0.6.2 -> 0.6.4

[bgamari]

This updates the previously-broken vllm derivation to 0.6.4 and brings it into a functional state. Specifically I:

• introduce needed Python dependencies (compressed-tensors, mistral-common, and partial-json-parser)
• rework how target selection is handled and introduce support for the CPU backend
• fix vllm's subprocess spawn logic to ensure that PYTHONPATH is propagated to the child

Smoke-tested with:

let
  pkgs = import ./. {};

  cpu = pkgs.python3Packages.vllm;

  rocm = pkgs.python3Packages.vllm.override {
    torch = pkgs.python3Packages.torchWithRocm;
  };

  cuda = pkgs.python3Packages.vllm.override {
    torch = pkgs.python3Packages.torchWithCuda;
  };
in { inherit cpu rocm cuda; }

invoking bin/vllm serve facebook/opt-125m --chat-template ./template.jinja and then bin/vllm chat.

Currently rocm is broken due to torch but cpu works. cuda test is still running.

Things done

• Built on platform(s)
  • x86_64-linux (using the cpu target)
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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