IQ3_K_R4

[ikawrakow]

Adding IQ3_K with 4 interleaved rows.

We get very signifiant performance gains on ARM_NEON and more modest gains on AVX2/Zen4. Overall slower than other _R4 quants, which is expected as 3-bit quantization is always kind of slow.

Here is PP-512 for LLaMA-3.1-8B on Zen4 (Ryzen-7950X), ARM_NEON (M2-Max) and AVX2 (Ryzen-5975WX)

Platform	Threads	IQ3_K	IQ3_K_R4	Speedup
ARM_NEON	8	54.94 ± 0.79	93.83 ± 0.09	1.708
Zen4	16	180.13 ± 0.48	230.33 ± 0.13	1.279
AVX2	32	197.59 ± 0.43	253.36 ± 0.50	1.282

We get decent performance gains for TG as well.
Here results for TG-128 on LLaMA-3.1-8B with different numbers of threads:

Platform	Threads	IQ3_K	IQ3_K_R4	Speedup
ARM_NEON	2	5.84 ± 0.00	6.71 ± 0.05	1.149
	4	11.14 ± 0.00	12.83 ± 0.01	1.152
	8	20.59 ± 0.17	23.07 ± 0.16	1.120
Zen4	1	5.06 ± 0.00	5.64 ± 0.00	1.115
	2	9.58 ± 0.01	10.50 ± 0.01	1.096
	4	16.56 ± 0.05	16.77 ± 0.32	1.013
AVX2	2	4.45 ± 0.00	6.83 ± 0.00	1.535
	4	8.24 ± 0.00	12.51 ± 0.00	1.518
	8	14.59 ± 0.04	16.23 ± 0.00	1.112