cpu: introduce chunking for repack matmuls and enable matmul-id chunking

[max-krasnyansky]

Similar to #16829 and tested in tandem.

A very simple dynamic chunking mechanism for repack matmuls. Helps on platforms with significant performance difference between the CPU cores, and helps distribute the work better under load in general.
I tested on M4 Pro and a few Snapdragons but it should work on all platforms.

See the details below.
I included a trace with instrumented matmuls that shows how threads threads endup processing chunks.

Details

## M4 Pro

Before (no other load)
| model                  |       size |     params | backend    | ngl | threads | fa | dev   |   test |            t/s |
| ---------------------- | ---------: | ---------: | ---------- | --: | ------: | -: | ----- | -----: |  ------------: |
| gpt-oss 20B MXFP4 MoE  |  11.27 GiB |    20.91 B | Metal      |   0 |       6 |  1 | none  |  pp256 |   75.67 ± 0.43 |
| gpt-oss 20B MXFP4 MoE  |  11.27 GiB |    20.91 B | Metal      |   0 |       6 |  1 | none  |   tg64 |   56.13 ± 0.26 |

| model                  |       size |     params | backend    | ngl | threads | fa | dev   |   test |            t/s |
| ---------------------- | ---------: | ---------: | ---------- | --: | ------: | -: | ----- | -----: | -------------: |
| llama 3B Q4_0          |   1.78 GiB |     3.21 B | Metal      |   0 |       2 |  1 | none  |  pp256 |  100.81 ± 2.22 |
| llama 3B Q4_0          |   1.78 GiB |     3.21 B | Metal      |   0 |       2 |  1 | none  |   tg64 |   37.27 ± 1.06 |
| llama 3B Q4_0          |   1.78 GiB |     3.21 B | Metal      |   0 |       4 |  1 | none  |  pp256 |  198.34 ± 0.21 |
| llama 3B Q4_0          |   1.78 GiB |     3.21 B | Metal      |   0 |       4 |  1 | none  |   tg64 |   67.88 ± 0.63 |
| llama 3B Q4_0          |   1.78 GiB |     3.21 B | Metal      |   0 |       6 |  1 | none  |  pp256 |  275.03 ± 8.60 |
| llama 3B Q4_0          |   1.78 GiB |     3.21 B | Metal      |   0 |       6 |  1 | none  |   tg64 |   92.09 ± 1.40 |

After (no other load)
| model                  |       size |     params | backend    | ngl | threads | fa | dev   |   test |            t/s |
| ---------------------- | ---------: | ---------: | ---------- | --: | ------: | -: | ----- | -----: |  ------------: |
| gpt-oss 20B MXFP4 MoE  |  11.27 GiB |    20.91 B | Metal      |   0 |       6 |  1 | none  |  pp256 |   76.57 ± 0.15 |
| gpt-oss 20B MXFP4 MoE  |  11.27 GiB |    20.91 B | Metal      |   0 |       6 |  1 | none  |   tg64 |   55.66 ± 0.46 |

| model                  |       size |     params | backend    | ngl | threads | fa | dev   |   test |            t/s |
| ---------------------  | ---------: | ---------: | ---------- | --: | ------: | -: | ----- | -----: | -------------: |
| llama 3B Q4_0          |   1.78 GiB |     3.21 B | Metal      |   0 |       2 |  1 | none  |  pp256 |  105.01 ± 0.33 |
| llama 3B Q4_0          |   1.78 GiB |     3.21 B | Metal      |   0 |       2 |  1 | none  |   tg64 |   38.63 ± 0.10 |
| llama 3B Q4_0          |   1.78 GiB |     3.21 B | Metal      |   0 |       4 |  1 | none  |  pp256 |  198.66 ± 0.19 |
| llama 3B Q4_0          |   1.78 GiB |     3.21 B | Metal      |   0 |       4 |  1 | none  |   tg64 |   67.40 ± 0.29 |
| llama 3B Q4_0          |   1.78 GiB |     3.21 B | Metal      |   0 |       6 |  1 | none  |  pp256 |  290.01 ± 1.31 |
| llama 3B Q4_0          |   1.78 GiB |     3.21 B | Metal      |   0 |       6 |  1 | none  |   tg64 |   89.92 ± 0.09 |


Chunking in action (no load)
thread-5: matmul ffn_up-0 nchunks 4 usec 7219
thread-3: matmul ffn_up-0 nchunks 4 usec 7221
thread-2: matmul ffn_up-0 nchunks 4 usec 7232
thread-1: matmul ffn_up-0 nchunks 4 usec 7247
thread-0: matmul ffn_up-0 nchunks 4 usec 7259
thread-4: matmul ffn_up-0 nchunks 4 usec 7260
thread-3: matmul ffn_out-0 nchunks 4 usec 7402
thread-1: matmul ffn_out-0 nchunks 4 usec 7423
thread-2: matmul ffn_out-0 nchunks 4 usec 7425
thread-4: matmul ffn_out-0 nchunks 4 usec 7402
thread-0: matmul ffn_out-0 nchunks 4 usec 7411
thread-5: matmul ffn_out-0 nchunks 4 usec 7402

Chunking in action (heavy other load)
thread-3: matmul ffn_up-6 nchunks 3 usec 8080
thread-1: matmul ffn_up-6 nchunks 5 usec 9055
thread-4: matmul ffn_up-6 nchunks 5 usec 9070
thread-5: matmul ffn_up-6 nchunks 5 usec 9428
thread-2: matmul ffn_up-6 nchunks 3 usec 9502
thread-0: matmul ffn_up-6 nchunks 3 usec 10552
thread-3: matmul ffn_out-6 nchunks 4 usec 8556
thread-0: matmul ffn_out-6 nchunks 3 usec 8612
thread-4: matmul ffn_out-6 nchunks 4 usec 8809
thread-1: matmul ffn_out-6 nchunks 5 usec 9275
thread-5: matmul ffn_out-6 nchunks 5 usec 9750
thread-2: matmul ffn_out-6 nchunks 3 usec 9963


## Snapdragon 8-Elite Gen5

## LLama3.2 1B Q4_0
  llama_model_loader: - type  f32:   34 tensors
  llama_model_loader: - type q4_0:  112 tensors
  llama_model_loader: - type q6_K:    1 tensors

Before
| model          |       size |     params | backend    | ngl | threads | fa | dev   |   test |            t/s |
| -------------- | ---------: | ---------: | ---------- | --: | ------: | -: | ----- | -----: | -------------: |
| llama 1B Q4_0  | 727.75 MiB |     1.24 B | CPU        |   0 |       2 |  1 | none  |  pp128 |  384.94 ± 9.15 |
| llama 1B Q4_0  | 727.75 MiB |     1.24 B | CPU        |   0 |       2 |  1 | none  |   tg64 |   65.17 ± 1.49 |
| llama 1B Q4_0  | 727.75 MiB |     1.24 B | CPU        |   0 |       4 |  1 | none  |  pp128 |  351.52 ± 0.28 |
| llama 1B Q4_0  | 727.75 MiB |     1.24 B | CPU        |   0 |       4 |  1 | none  |   tg64 |   71.00 ± 1.49 |
| llama 1B Q4_0  | 727.75 MiB |     1.24 B | CPU        |   0 |       6 |  1 | none  |  pp128 |  512.93 ± 1.78 |
| llama 1B Q4_0  | 727.75 MiB |     1.24 B | CPU        |   0 |       6 |  1 | none  |   tg64 |   77.26 ± 1.29 |


After
| model          |       size |     params | backend    | ngl | threads | fa | dev   |    test |            t/s |
| -------------- | ---------: | ---------: | ---------- | --: | ------: | -: | ----- | ------: |--------------: |
| llama 1B Q4_0  | 727.75 MiB |     1.24 B | CPU        |   0 |       2 |  1 | none  |   pp128 |  395.65 ± 7.81 |
| llama 1B Q4_0  | 727.75 MiB |     1.24 B | CPU        |   0 |       2 |  1 | none  |    tg64 |   64.40 ± 0.85 |
| llama 1B Q4_0  | 727.75 MiB |     1.24 B | CPU        |   0 |       4 |  1 | none  |   pp128 |  459.51 ± 1.04 |
| llama 1B Q4_0  | 727.75 MiB |     1.24 B | CPU        |   0 |       4 |  1 | none  |    tg64 |   73.62 ± 0.67 |
| llama 1B Q4_0  | 727.75 MiB |     1.24 B | CPU        |   0 |       6 |  1 | none  |   pp128 |  669.03 ± 1.41 |
| llama 1B Q4_0  | 727.75 MiB |     1.24 B | CPU        |   0 |       6 |  1 | none  |    tg64 |   79.75 ± 0.56 |


## Llama3.2 3B Q4_0
  llama_model_loader: - type  f32:   58 tensors
  llama_model_loader: - type q4_0:  196 tensors
  llama_model_loader: - type q6_K:    1 tensors

Before
| model           |       size |     params | backend    | ngl | threads | fa | dev   |  test |             t/s |
| --------------- | ---------: | ---------: | ---------- | --: | ------: | -: | ----- | ----: | --------------: |
| llama 3B Q4_0   |   1.78 GiB |     3.21 B | CPU        |   0 |       2 |  1 | none  | pp128 |   127.73 ± 2.43 |
| llama 3B Q4_0   |   1.78 GiB |     3.21 B | CPU        |   0 |       2 |  1 | none  |  tg64 |    27.91 ± 0.61 |
| llama 3B Q4_0   |   1.78 GiB |     3.21 B | CPU        |   0 |       4 |  1 | none  | pp128 |   122.97 ± 0.02 |
| llama 3B Q4_0   |   1.78 GiB |     3.21 B | CPU        |   0 |       4 |  1 | none  |  tg64 |    29.72 ± 1.09 |
| llama 3B Q4_0   |   1.78 GiB |     3.21 B | CPU        |   0 |       6 |  1 | none  | pp128 |  159.59 ± 14.06 |
| llama 3B Q4_0   |   1.78 GiB |     3.21 B | CPU        |   0 |       6 |  1 | none  |  tg64 |    30.33 ± 0.60 |


After
| model           |       size |     params | backend    | ngl | threads | fa | dev   |  test |             t/s |
| --------------- | ---------: | ---------: | ---------- | --: | ------: | -: | ----- | ----: | --------------: |
| llama 3B Q4_0   |   1.78 GiB |     3.21 B | CPU        |   0 |       2 |  1 | none  | pp128 |   128.16 ± 2.09 |
| llama 3B Q4_0   |   1.78 GiB |     3.21 B | CPU        |   0 |       2 |  1 | none  |  tg64 |    27.46 ± 0.47 |
| llama 3B Q4_0   |   1.78 GiB |     3.21 B | CPU        |   0 |       4 |  1 | none  | pp128 |   161.89 ± 0.30 |
| llama 3B Q4_0   |   1.78 GiB |     3.21 B | CPU        |   0 |       4 |  1 | none  |  tg64 |    30.07 ± 0.65 |
| llama 3B Q4_0   |   1.78 GiB |     3.21 B | CPU        |   0 |       6 |  1 | none  | pp128 |   227.02 ± 7.26 |
| llama 3B Q4_0   |   1.78 GiB |     3.21 B | CPU        |   0 |       6 |  1 | none  |  tg64 |    32.16 ± 0.58 |


## Llama3.2 1B chunking in action

thread-0: matmul ffn_up-11 nchunks 7 usec 143
thread-5: matmul ffn_up-11 nchunks 8 usec 147
thread-3: matmul ffn_up-11 nchunks 2 usec 150
thread-1: matmul ffn_up-11 nchunks 2 usec 150
thread-2: matmul ffn_up-11 nchunks 2 usec 152
thread-4: matmul ffn_up-11 nchunks 3 usec 158
thread-0: matmul ffn_out-11 nchunks 7 usec 124
thread-1: matmul ffn_out-11 nchunks 2 usec 125
thread-5: matmul ffn_out-11 nchunks 8 usec 128
thread-4: matmul ffn_out-11 nchunks 2 usec 129
thread-2: matmul ffn_out-11 nchunks 2 usec 139
thread-3: matmul ffn_out-11 nchunks 3 usec 150


## Galaxy S25+ (Snapdragon 8-Elite Gen4)

## LLama3.2 1B chunking in action

thread-2: matmul ffn_up-11 nchunks 6 usec 147
thread-3: matmul ffn_up-11 nchunks 3 usec 150
thread-0: matmul ffn_up-11 nchunks 6 usec 147
thread-5: matmul ffn_up-11 nchunks 3 usec 150
thread-1: matmul ffn_up-11 nchunks 3 usec 147
thread-4: matmul ffn_up-11 nchunks 3 usec 152
thread-4: matmul ffn_out-11 nchunks 3 usec 136
thread-2: matmul ffn_out-11 nchunks 6 usec 142
thread-5: matmul ffn_out-11 nchunks 3 usec 146
thread-1: matmul ffn_out-11 nchunks 3 usec 136
thread-0: matmul ffn_out-11 nchunks 6 usec 144
thread-3: matmul ffn_out-11 nchunks 3 usec 136

[max-krasnyansky]

@slaren any objections to merging this?

[max-krasnyansky]

@slaren Thanks for approving the PR!
I'm thinking of updating the original MatMul and MatMul-ID chunking to use the same logic for computing the chunk size.
i.e Instead of the arbitrary "64 or 16 rows chunk_size" we aim for 4x chunks per thread to take care of any imbalance in the CPUs either simply due ext load or big.LITTLE.
Merging this and will iterate in another PR.