metal: deepseek v4 optimize token generation

[tarruda]

Overview

Picking a couple of commits from my branch which improve DSv4 token generation

Additional information

This builds on ggml-org#26512 and will improve token generation and keep it more stable as context grows. Here's -d 0,10000,20000,30000 -n 128 -p 2048 -b 2048 -ub 2048 on a M1 Ultra:

| model                          |       size |     params | backend    | threads | n_ubatch |  fa |            test |                  t/s |
| ------------------------------ | ---------: | ---------: | ---------- | ------: | -------: | --: | --------------: | -------------------: |
| deepseek4 ?B Q8_0              |  91.94 GiB |   284.33 B | MTL,BLAS   |      16 |     2048 |   1 |          pp2048 |        199.61 ± 0.96 |
| deepseek4 ?B Q8_0              |  91.94 GiB |   284.33 B | MTL,BLAS   |      16 |     2048 |   1 |           tg128 |         25.74 ± 0.03 |
| deepseek4 ?B Q8_0              |  91.94 GiB |   284.33 B | MTL,BLAS   |      16 |     2048 |   1 | pp2048 @ d10000 |        190.85 ± 0.65 |
| deepseek4 ?B Q8_0              |  91.94 GiB |   284.33 B | MTL,BLAS   |      16 |     2048 |   1 |  tg128 @ d10000 |         22.82 ± 0.02 |
| deepseek4 ?B Q8_0              |  91.94 GiB |   284.33 B | MTL,BLAS   |      16 |     2048 |   1 | pp2048 @ d20000 |        182.61 ± 0.75 |
| deepseek4 ?B Q8_0              |  91.94 GiB |   284.33 B | MTL,BLAS   |      16 |     2048 |   1 |  tg128 @ d20000 |         22.37 ± 0.04 |
| deepseek4 ?B Q8_0              |  91.94 GiB |   284.33 B | MTL,BLAS   |      16 |     2048 |   1 | pp2048 @ d30000 |        175.11 ± 0.27 |
| deepseek4 ?B Q8_0              |  91.94 GiB |   284.33 B | MTL,BLAS   |      16 |     2048 |   1 |  tg128 @ d30000 |         22.09 ± 0.08 |

Requirements

• I have read and agree with the contributing guidelines
• AI usage disclosure: Yes, 100% GPT 5.6 Sol. I'm not a ML kernel programmer and have no idea what I'm doing. If anyone wants to create a new PR that cleans up, improves or rewrite these changes, they have my blessing to do so.