Fixed tokenizer.model not found error when model dir is symlink

[mqy]

In convert-pth-to-ggml.py, dir_model is something like models/7B or models/7B/. tokenizer.model is expected under model's parent dir.

When dir_model is a symlink, f"{dir_model}/../tokenizer.model" would not be found. Let's use the model's parent dir directly.