feat: select vulkan device with env variable (#629)

Author: stduhpf

stable-diffusion.cpp

@@ -165,7 +165,27 @@ class StableDiffusionGGML {
 #endif
 #ifdef SD_USE_VULKAN
         LOG_DEBUG("Using Vulkan backend");
-        for (int device = 0; device < ggml_backend_vk_get_device_count(); ++device) {
+        size_t device          = 0;
+        const int device_count = ggml_backend_vk_get_device_count();
+        if (device_count) {
+            const char* SD_VK_DEVICE = getenv("SD_VK_DEVICE");
+            if (SD_VK_DEVICE != nullptr) {
+                std::string sd_vk_device_str = SD_VK_DEVICE;
+                try {
+                    device = std::stoull(sd_vk_device_str);
+                } catch (const std::invalid_argument&) {
+                    LOG_WARN("SD_VK_DEVICE environment variable is not a valid integer (%s). Falling back to device 0.", SD_VK_DEVICE);
+                    device = 0;
+                } catch (const std::out_of_range&) {
+                    LOG_WARN("SD_VK_DEVICE environment variable value is out of range for `unsigned long long` type (%s). Falling back to device 0.", SD_VK_DEVICE);
+                    device = 0;
+                }
+                if (device >= device_count) {
+                    LOG_WARN("Cannot find targeted vulkan device (%llu). Falling back to device 0.", device);
+                    device = 0;
+                }
+            }
+            LOG_INFO("Vulkan: Using device %llu", device);
             backend = ggml_backend_vk_init(device);
         }
         if (!backend) {