Feature Request: Q6_0 quant #10848

Prerequisites

• I am running the latest code. Mention the version if possible as well.
• I carefully followed the README.md.
• I searched using keywords relevant to my issue to make sure that I am creating a new issue that is not already open (or closed).
• I reviewed the Discussions, and have a new and useful enhancement to share.

Feature Description

Qwen 2 and models can't use Q5_K and Q6_K ggml_types to quantize ffn_down due to its irregular shape.
It thus brings us on Q5_1, which is now suboptimal, and sometimes subpar to Q5_0. Or to Q8_0, much bigger.

Ikawrakow published a while ago a Q6_0 which can quantize irregularly shaped tensors, and it would provide a great alternative to either Q5_0 or Q8_0.

After all, ffn_down represents something like 25% of the weight of a layer, and so, for Qwen 2 models, that means 0.4 to 0.5bpw more for a Q6_K quantized model. In Q5_K_S or Q5_K_M, Q5_1 tensors are also bigger than Q5_K, with a lesser quality.

In all these cases, a Q6_0 ggml_type would be more appropriate.

Ikawrakow has already one made on his fork. It'd be great to see such a quant in LlamaCPP mainline.

Motivation

Better ratio quality/size for models with irregularly shaped tensors.

Possible Implementation

Factor and merge IK's Q6_0 GGML_Type.
Or
Develop and implement an equivalent here.