UPSTREAM PR #18919: metal : support virtual devices

[loci-dev]

Note

Source pull request: ggml-org/llama.cpp#18919

Support virtual Metal devices. Allows simulating multi-GPU environments on Mac using the new GGML_METAL_DEVICES environment variable.

GGML_METAL_DEVICES=4 ./bin/llama-completion -m [model.gguf]

...

0.02.020.033 I llama_memory_breakdown_print: | memory breakdown [MiB]    |  total     free    self   model   context   compute    unaccounted |
0.02.020.034 I llama_memory_breakdown_print: |   - MTL0 (Apple M2 Ultra) | 165150 = 158091 + (1916 =   780 +    1024 +     112) +        5143 |
0.02.020.034 I llama_memory_breakdown_print: |   - MTL1 (Apple M2 Ultra) | 165150 = 158091 + (1738 =   780 +     896 +      62) +        5320 |
0.02.020.036 I llama_memory_breakdown_print: |   - MTL2 (Apple M2 Ultra) | 165150 = 158091 + (1198 =   240 +     896 +      62) +        5861 |
0.02.020.037 I llama_memory_breakdown_print: |   - MTL3 (Apple M2 Ultra) | 165150 = 158091 + (2205 =  1137 +     768 +     300) +        4853 |
0.02.020.037 I llama_memory_breakdown_print: |   - Host                  |                     364 =   296 +       0 +      68                |

[loci-review]

No meaningful performance changes were detected across 115327 analyzed functions in the following binaries: build.bin.libllama.so, build.bin.llama-cvector-generator, build.bin.llama-tts, build.bin.libmtmd.so, build.bin.llama-bench, build.bin.libggml-base.so, build.bin.libggml-cpu.so, build.bin.libggml.so, build.bin.llama-tokenize, build.bin.llama-qwen2vl-cli, build.bin.llama-quantize, build.bin.llama-gguf-split, build.bin.llama-llava-cli, build.bin.llama-minicpmv-cli, build.bin.llama-gemma3-cli.

🔎 Full breakdown: Loci Inspector.
💬 Questions? Tag @loci-dev.

[loci-review]

No meaningful performance changes were detected across 115327 analyzed functions in the following binaries: build.bin.libllama.so, build.bin.llama-cvector-generator, build.bin.libmtmd.so, build.bin.llama-tts, build.bin.libggml-cpu.so, build.bin.libggml.so, build.bin.libggml-base.so, build.bin.llama-tokenize, build.bin.llama-quantize, build.bin.llama-qwen2vl-cli, build.bin.llama-bench, build.bin.llama-gemma3-cli, build.bin.llama-gguf-split, build.bin.llama-llava-cli, build.bin.llama-minicpmv-cli.

🔎 Full breakdown: Loci Inspector.
💬 Questions? Tag @loci-dev.