Metal: much faster MoE prompt processing (#307)

* MoE improvements on Metal

This version beats mainline, there are things I don't understand:
* Mianline has effectively gone to GEMV for MUL_MAT_ID. We can do the
  same, but we are 30% slower. Why?
* Using actual GEMM, we beat mainline with ubtach size of 128. But then
  performance degrades. Why?

* Some cleanup

* Much better

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Co-authored-by: Iwan Kawrakow <[email protected]>

Author: ikawrakow