Llama-quantize: Partial requant feature

[Nexesenex]

Inspired by the recently added --dry-run option for llama-quantize.

This PR allows to partially requantize a split quantized .gguf by requantizing only the missing splits in the destination directory. (useful for whoever is used to change certains tensors' quantization to improve an overall quant strategy: just delete the splits you want to requantize in the destination directory)

It works both for GGUF which are split tensor by tensor, or by group of several tensors. (though this one is not very much tested except with 2 tensors by split: I'm myself using directories of single tensors GGUFs since @Thireus made his GGUF-Tool-Suite)

It also adds automatic directory creation for both llama-quantize and gguf-split in case the destination directory of the quantization/split doesn't exist. (A longstanding lacking feature ^^)

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[ikawrakow]

@Nexesenex

Precisely what do you mean by "copyported"? "Copyported" from where? Considering that I wrote most of the llama-quantize code in llama.cpp, do you seriously believe that I need to "copyport" somebody else's modifications?

[ikawrakow]

Please merge and resolve conflicts so I don't need to be reviewing the --dry-run changes along with the actual changes of the PR.

[Nexesenex]

Precisely what do you mean by "copyported"? "Copyported" from where? Considering that I wrote most of the llama-quantize code in llama.cpp, do you seriously believe that I need to "copyport" somebody else's modifications?

I apologize, I simply meant that I saw an equivalent PR on llama.cpp, and saw a recent conversation when the terms "copying" and "porting" were opposed. I merged both terms as I did in the initial thread, forgetting that applied to what I would do, not to what you would do (rewriting it fully by yourself).

Now, you know very well that I don't believe that you NEED to "copyport" somebody's else modifications considering that you indeed wrote most of the llama-quantize code of llama.cpp, and that I didn't imply that precise statement. But what goes without saying goes better by saying it. I apologize again for the misunderstanding born of my inadequate terminology, it bore no malice.

Then, the merger is done, and I'm currently correcting my PR accordingly to your review.