metal : refactor mat-vec code (#12569)
* metal : refactor mat-vec code
ggml-ci
* metal : rename all_sum -> sum_all
ggml-ci
* metal : fix comments [no ci]
* metal : fix nr constant [no ci]
* metal : mv q6_K support nr0 > 1
ggml-ci
* metal : reduce register pressure
ggml-ci
* metal : fix typo [no ci]
* metal : reduce register pressure
ggml-ci