common : add --sched-n-copies parameter for pipeline parallelism configuration

[mxxm-t]

Replace compile-time GGML_SCHED_MAX_COPIES with runtime configuration. Add --sched-n-copies parameter to control scheduler input copies (default: 4, max: 16). Implement ggml_backend_sched_set_n_copies() to override the number of copies used for parallel execution. Update llama-bench to support the new parameter.

Changing this parameter can give quiet a bit of performance.

Benchmarks

| model                          |       size |     params | backend    | ngl | sched_n_copies | n_ubatch | fa | mmap | dio |            test |                  t/s |
| ------------------------------ | ---------: | ---------: | ---------- | --: | -------------: | -------: | -: | ---: | --: | --------------: | -------------------: |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |              4 |     1024 |  1 |    0 |   1 |          pp1024 |        641.02 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |              4 |     1024 |  1 |    0 |   1 |          pp2048 |       1111.21 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |              4 |     1024 |  1 |    0 |   1 |          pp8096 |       2053.24 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |              4 |     1024 |  1 |    0 |   1 |         pp16384 |       2165.73 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |              4 |     1024 |  1 |    0 |   1 |         pp32768 |       2112.82 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |              4 |     1024 |  1 |    0 |   1 |         pp65536 |       1792.52 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |              4 |     1024 |  1 |    0 |   1 |        pp131072 |       1316.19 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |              8 |     1024 |  1 |    0 |   1 |          pp1024 |        636.88 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |              8 |     1024 |  1 |    0 |   1 |          pp2048 |       1110.96 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |              8 |     1024 |  1 |    0 |   1 |          pp8096 |       2336.29 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |              8 |     1024 |  1 |    0 |   1 |         pp16384 |       2705.89 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |              8 |     1024 |  1 |    0 |   1 |         pp32768 |       2705.33 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |              8 |     1024 |  1 |    0 |   1 |         pp65536 |       2313.66 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |              8 |     1024 |  1 |    0 |   1 |        pp131072 |       1698.53 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |             16 |     1024 |  1 |    0 |   1 |          pp1024 |        641.25 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |             16 |     1024 |  1 |    0 |   1 |          pp2048 |       1100.32 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |             16 |     1024 |  1 |    0 |   1 |          pp8096 |       2355.55 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |             16 |     1024 |  1 |    0 |   1 |         pp16384 |       2699.75 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |             16 |     1024 |  1 |    0 |   1 |         pp32768 |       2706.86 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |             16 |     1024 |  1 |    0 |   1 |         pp65536 |       2313.20 ± 0.00 |
| gpt-oss 120B MXFP4 MoE         |  59.02 GiB |   116.83 B | ROCm       |  99 |             16 |     1024 |  1 |    0 |   1 |        pp131072 |       1699.27 ± 0.00 |

[mxxm-t]

More tests on 10x MI50 32GB:

Qwen3-4B-Instruct-2507-Q4_0

| model                          |       size |     params | backend    | ngl | sched_n_copies | n_ubatch | fa | mmap | dio |            test |                  t/s |
| ------------------------------ | ---------: | ---------: | ---------- | --: | -------------: | -------: | -: | ---: | --: | --------------: | -------------------: |
| qwen3 4B Q4_0                  |   2.21 GiB |     4.02 B | ROCm       |  99 |              4 |     1024 |  1 |    0 |   1 |         pp16384 |       4055.10 ± 0.00 |
| qwen3 4B Q4_0                  |   2.21 GiB |     4.02 B | ROCm       |  99 |              8 |     1024 |  1 |    0 |   1 |         pp16384 |       4956.52 ± 0.00 |
| qwen3 4B Q4_0                  |   2.21 GiB |     4.02 B | ROCm       |  99 |             16 |     1024 |  1 |    0 |   1 |         pp16384 |       4951.32 ± 0.00 |

Qwen3-14B-Q8_0

| model                          |       size |     params | backend    | ngl | sched_n_copies | n_ubatch | fa | mmap | dio |            test |                  t/s |
| ------------------------------ | ---------: | ---------: | ---------- | --: | -------------: | -------: | -: | ---: | --: | --------------: | -------------------: |
| qwen3 14B Q8_0                 |  14.61 GiB |    14.77 B | ROCm       |  99 |              4 |     1024 |  1 |    0 |   1 |         pp16384 |        952.61 ± 0.00 |
| qwen3 14B Q8_0                 |  14.61 GiB |    14.77 B | ROCm       |  99 |              8 |     1024 |  1 |    0 |   1 |         pp16384 |       1226.22 ± 0.00 |
| qwen3 14B Q8_0                 |  14.61 GiB |    14.77 B | ROCm       |  99 |             16 |     1024 |  1 |    0 |   1 |         pp16384 |       1224.99 ± 0.00 |

MiniMax-M2.1-GGUF_Q8_0

| model                          |       size |     params | backend    | ngl | sched_n_copies | n_ubatch | fa | mmap | dio |            test |                  t/s |
| ------------------------------ | ---------: | ---------: | ---------- | --: | -------------: | -------: | -: | ---: | --: | --------------: | -------------------: |
| minimax-m2 230B.A10B Q8_0      | 226.43 GiB |   228.69 B | ROCm       |  99 |              4 |     1024 |  1 |    0 |   1 |         pp16384 |        654.39 ± 0.00 |
| minimax-m2 230B.A10B Q8_0      | 226.43 GiB |   228.69 B | ROCm       |  99 |              8 |     1024 |  1 |    0 |   1 |         pp16384 |        881.94 ± 0.00 |
| minimax-m2 230B.A10B Q8_0      | 226.43 GiB |   228.69 B | ROCm       |  99 |             16 |     1024 |  1 |    0 |   1 |         pp16384 |        878.99 ± 0.00 |

Qwen3.5-35B-A3B-UD-Q8_K_XL

| model                          |       size |     params | backend    | ngl | sched_n_copies | n_ubatch | fa | mmap | dio |            test |                  t/s |
| ------------------------------ | ---------: | ---------: | ---------- | --: | -------------: | -------: | -: | ---: | --: | --------------: | -------------------: |
| qwen35moe 35B.A3B Q8_0         |  36.03 GiB |    34.66 B | ROCm       |  99 |              4 |     1024 |  1 |    0 |   1 |         pp16384 |       1951.05 ± 0.00 |
| qwen35moe 35B.A3B Q8_0         |  36.03 GiB |    34.66 B | ROCm       |  99 |              8 |     1024 |  1 |    0 |   1 |         pp16384 |       2518.78 ± 0.00 |
| qwen35moe 35B.A3B Q8_0         |  36.03 GiB |    34.66 B | ROCm       |  99 |             16 |     1024 |  1 |    0 |   1 |         pp16384 |       2514.52 ± 0.00 |

Qwen3.5-122B-A10B-UD-Q8_K_XL

| model                          |       size |     params | backend    | ngl | sched_n_copies | n_ubatch | fa | mmap | dio |            test |                  t/s |
| ------------------------------ | ---------: | ---------: | ---------- | --: | -------------: | -------: | -: | ---: | --: | --------------: | -------------------: |
| qwen35moe 122B.A10B Q8_0       | 159.10 GiB |   122.11 B | ROCm       |  99 |              4 |     1024 |  1 |    0 |   1 |         pp16384 |        209.00 ± 0.00 |
| qwen35moe 122B.A10B Q8_0       | 159.10 GiB |   122.11 B | ROCm       |  99 |              8 |     1024 |  1 |    0 |   1 |         pp16384 |        209.61 ± 0.00 |