FlashMLA-2 (CPU): faster and smaller compute buffer size

[ikawrakow]

This PR improves the CPU implementation of FlashMLA in 3 ways:

• Faster prompt processing - about 13% improvement for a context of 16k tokens
• Smaller compute buffer size - about 60% reduction for a context of 128k tokens

To recall, FlashMLA-2 is enabled via -mla 2 -fa, and is the variant that works on the CPU and on CUDA.

The improvement is achieved by adding implementations for

• ggml_mul_mat where the second operand is not fp32
• ggml_concat where the operands are quantized
• ggml_repeat where the operand is not fp32

This allows us to avoid conversions to fp32 that can become quite costly when operating on a very large context.

Here is a PP performance comparison for DeepSeek-Lite running on a Ryzen-7950X CPU between the main branch and this PR

model	test	t/s (main)	t/s (PR)	Speedup
deepseek2 16B IQ4_NL	pp512	668.46 ± 1.74	680.74 ± 21.47	1.018
deepseek2 16B IQ4_NL	pp1024	646.86 ± 0.94	668.65 ± 0.44	1.034
deepseek2 16B IQ4_NL	pp2048	596.56 ± 1.70	628.99 ± 1.72	1.054
deepseek2 16B IQ4_NL	pp4096	513.16 ± 1.42	552.36 ± 4.61	1.076
deepseek2 16B IQ4_NL	pp8192	398.45 ± 3.51	442.89 ± 3.96	1.112
deepseek2 16B IQ4_NL	pp16384	272.58 ± 7.06	308.21 ± 5.91	1.131

And here is a comparison between compute buffer sizes along with KV cache size for fp16 cache

context	KV cache size (MiB)	compute buffer (MiB, PR)	compute buffer (MiB, main)
2048	60.75	204.00	204.00
4096	121.50	204.00	204.00
8192	243.00	220.01	358.01
16384	486.00	452.01	712.01
32768	972.00	884.01	1404.02
65536	1944.00	1748.01	2788.02
131072	3888.00	3476.02	5556.02

I did a quick attempt to also implement on CUDA, but something wasn't working, so left it for a future PR. This also implies that the new way of preparing the compute graph will only be used if the code was built without support for additional back-ends (even if zero layers are uploaded to them, to avoid fighting with the back-end scheduler).

[davidsyoung]

Nice! The compute buffer on CUDA makes it hard to balance model layers with the compute buffer, so when you manage to get CUDA implementation working it'll be amazing. Thank you for your work on this