metal : use function constants for mul_mv_ext kernels

[ggerganov]

• Make NSG and nxpsg compile-time constants
• Remove NW template arg
• Tune some mat-vec constants

Model	Test	t/s master	t/s gg/metal-mul-mv-ext-opt	Speedup
qwen2 14B Q4_K_M	tg32	54.54	55.61	1.02
qwen2 32B IQ4_NL - 4.5 bpw	tg32	28.20	28.27	1.00
qwen2 32B Q4_K_M	tg32	27.33	27.51	1.01
qwen2 32B Q5_K_M	tg32	21.18	21.47	1.01
qwen2 32B Q6_K	tg32	21.25	22.97	1.08
qwen2 3B IQ4_NL - 4.5 bpw	tg32	155.64	157.27	1.01
qwen2 3B Q4_K_M	tg32	148.75	155.12	1.04
qwen2 3B Q5_K_M	tg32	130.03	131.54	1.01
qwen2 3B Q6_K	tg32	136.94	143.11	1.05
qwen2 7B IQ4_NL - 4.5 bpw	tg32	108.95	109.78	1.01
qwen2 7B Q4_K_M	tg32	100.48	103.56	1.03
qwen2 7B Q5_K_M	tg32	78.64	79.69	1.01
qwen2 7B Q6_K	tg32	84.44	90.19	1.07