Function config_from_gguf_dir 

pub fn config_from_gguf_dir(model_dir: &Path) -> Result<ModelConfig>

Build a ModelConfig from the .gguf in model_dir, with no config.json.

vocab is taken from the token_embd.weight shape (ggml dims are [embedding_length, vocab], so the trailing dim is the vocab), and tied embeddings are inferred from the absence of an output.weight tensor.