add support of codeqwen due to tokenizer

[JustinLin610]

we just released codeqwen1.5, see blog and model for more info.

since for codeqwen1.5, we use a different tokenizer, which is based on sentencepiece, we need to make some changes for the convert-hf-to-gguf.py for the adaptation.

[JustinLin610]

@ggerganov feel free to take a look at this small code change 😃