feat: fill-in-middle support (#1386)

* Proper fill-in-middle support

Use prefix/middle/suffix tokens when metadata is present in GGUF, like f.ex. in [this](https://huggingface.co/CISCai/CodeQwen1.5-7B-Chat-SOTA-GGUF) one.

* fall back to internal prefix/middle/suffix id

In some cases llama.cpp will make a guess at fim tokens, use them if there's no metadata.

* typo--

* don't insert special tokens that are not there in suffix

Note: add_bos is misnamed, it's actually add_special and can cause several special tokens to be added to the token list (the special parameter is actually parse_special).

* don't add/parse any special tokens when using fim

I've left original behavior when no fim tokens are found, but this should perhaps be re-evaluated.

* don't append suffix to prompt_tokens unless fim tokens are detected

* make sure we only do this for fim

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Co-authored-by: Andrei <abetlen@gmail.com>