Llama-quantize: Partial requant feature (#1313)

* 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

src/llama.cpp

@@ -4414,6 +4414,7 @@ struct llama_model_quantize_params llama_model_quantize_default_params() {
         /*.ignore_imatrix_rules        =*/ false,
         /*.only_repack                 =*/ false,
         /*.dry_run                     =*/ false,
+        /*.partial_requant             =*/ false,
         /*.imatrix                     =*/ nullptr,
         /*.kv_overrides                =*/ nullptr,
         /*.custom_quants               =*/ nullptr,