* Partial Requant feature for llama-quantize
- Inspired by the recently portcopied --dry-run feature.
- Allows to partially requantize a split quantized .gguf by requantizing only the missing splits in the destination directory.
- Works both for GGUF which are split tensors by tensors, or by group of several tensors (though this one is not very much tested beyond 2 tensors by split).
- Vibe coded.
* Create output directory if it doesn't exist in llama-quantize
* Create output directory if it doesn't exist in gguf-split
* Add exit when directory fails to be created on Windows
* Use std::filesystem
* cleanup
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