metal : add Q2_K implementation (#1762)

* metal : add Q2_K implementation

27.1 ms / token on M2 Max 30-core GPU, so about the
same speed as Q4_0. Memory throughput is ~156 GB/s.

The access pattern used in the Q2_K
CUDA implementation resulted in significantly lower
performance (~31 ms/token).

* Fixing merge conflicts

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Co-authored-by: Iwan Kawrakow <iwan.kawrakow@gmail.com>