ggml : shorten virtual device naming in CUDA and Metal (#27608)

* ggml : shorten virtual device naming in CUDA and Metal

Assisted-by: llama.cpp:DeepSeek-V4-Flash-0731

* ggml-metal : build device description at init

Assisted-by: llama.cpp:DeepSeek-V4-Flash-0731

* cont : naming

ggml/src/ggml-cuda/ggml-cuda.cu

@@ -4611,8 +4611,8 @@ static std::string ggml_cuda_device_description(int device) {
     const ggml_cuda_device_info & info = ggml_cuda_info();
     std::string description = prop.name;
     if (info.device_count > info.physical_device_count) {
-        description += " (physical device " + std::to_string(info.devices[device].physical_device) +
-                       ", virtual device " + std::to_string(info.devices[device].virtual_index) + ")";
+        description += " (dev p" + std::to_string(info.devices[device].physical_device) +
+                       "/v" + std::to_string(info.devices[device].virtual_index) + ")";
     }
     return description;
 }