Convert: mixed k-quant with legacy quant fallback

[stduhpf]

Adds a new cli argument: --fallback-type.

If tensors cannot be quantized to a k-quant because of block size issues, the fallback type will be used instead of full precision.

Very useful for SD3.5 models, because 90% of SD3.5 8B weights can't be quantized to k quants.

--type q4_k --fallback-type q4_0 has always the exact same output size as --type q4_0, but with less degradation.

Somewhat adresses #446

[stduhpf]

I'm currently uploading quantized weights to HF, but with my cellular data, it takes very long.

here:

• https://huggingface.co/stduhpf/SD3.5-Large-Turbo-GGUF-mixed-sdcpp
• https://huggingface.co/stduhpf/SD3.5-Large-GGUF-mixed-sdcpp