Metal: much faster MoE prompt processing

[ikawrakow]

The prompt processing (PP) performance on Metal for MoE models with many experts (such as DeepSeek) is pathetic. Here, and also in mainline before the very recent PR 12612. This mainline PR brings PP performance to a more acceptable level by effectively using GEMV for matrix multiplications involving MoE tensors.

This PR does much better than that. On my M2-Max (30-core GPU) PP performance for DeepSeek-Lite is now 1.75X faster than mainline (build: a6f32f0b3 (5018)), and 5X compared to the main branch.

Also, on mainline I observe a very peculiar performance behavior as a function of u_batch:

model	size	backend	n_ubatch	test	t/s
deepseek2 16B Q8_0	15.55 GiB	Metal	128	pp512	254.43 ± 2.02
deepseek2 16B Q8_0	15.55 GiB	Metal	256	pp512	142.42 ± 0.24
deepseek2 16B Q8_0	15.55 GiB	Metal	512	pp512	417.56 ± 0.18

Interesting, right? For u_batch = 512 (where performance is maximized) the matrix multiplication is done using GEMV. For u_batch = 128, 256, it is done using GEMM, but in an extremely inefficient way, where the inefficiency increases with u_batch size, so performance degrades.

Here is what we get with this PR:

model	size	backend	n_ubatch	test	t/s
deepseek2 16B Q8_0	15.55 GiB	Metal	128	pp512	585.19 ± 1.07
deepseek2 16B Q8_0	15.55 GiB	Metal	256	pp512	685.58 ± 3.39
deepseek2 16B Q8_0	15.55 GiB	Metal	512	pp512	726.94 ± 2.35

The PR became much bigger than it should have been. But as TG performance is now slightly lower than mainline, and the only change that seemed promising to explain the difference was PR 9698, I decided to add that change. It made zero difference, but resulted in 2k lines of code moved around.