llama : support StableLM 2 1.6B (#5052)

* llama : support StableLM 2 1.6B

* convert : fix Qwen's set_vocab wrongly naming all special tokens [PAD{id}]

* convert : refactor Qwen's set_vocab to use it for StableLM 2 too

* nix : add tiktoken to llama-python-extra

* convert : use presence of tokenizer.json to determine StableLM tokenizer loader

It's a less arbitrary heuristic than the vocab size.

.devops/nix/package.nix

@@ -73,6 +73,7 @@ let
     ps: [
       ps.numpy
       ps.sentencepiece
+      ps.tiktoken
       ps.torchWithoutCuda
       ps.transformers
     ]