vulkan: add q2_K implementation in mul_mmq with ACC_TYPE_VEC2

[SavicStefan]

For q2_K, added the ACC_TYPE_VEC2 implementation, as seen in PR #16203.

Before (master) NVIDIA GeForce RTX 4060 Ti:
MUL_MAT(type_a=q2_K,type_b=f32,m=4096,n=512,k=14336,bs=[1,1],nr=[1,1],per=[0,1,2,3],k_v=0,o=1): 190 runs - 5292.84 us/run - 60.13 GFLOP/run - 11.36 TFLOPS

After (PR) NVIDIA GeForce RTX 4060 Ti:
MUL_MAT(type_a=q2_K,type_b=f32,m=4096,n=512,k=14336,bs=[1,1],nr=[1,1],per=[0,1,2,3],k_v=0,o=1):                258 runs -  3887.86 us/run -  60.13 GFLOP/run -  15.47 TFLOPS

Which is around +26% peformance increase on us/run.
I also need to try for other types.