ggml : add ggml_backend_load_variant to load specific backend variant

This commit adds a new function ggml_backend_load_variant to the ggml
library.

The motivation for this is that it is useful to be able to load a single
backend variant by name. The particular use case I have is for testing
where I want to be able to load a specific variant of a backend and then
find out what extra buffer types it may support and then load buffer
type using that backend variant.

Author: danbev

ggml/include/ggml-backend.h

@@ -245,6 +245,7 @@ extern "C" {
     GGML_API void               ggml_backend_load_all_from_path(const char * dir_path);
     // Load all variants for a backend and register them
     GGML_API void               ggml_backend_load_all_variants(const char * name);
+    GGML_API void               ggml_backend_load_variant(const char * name, const char * variant);
 
     //
     // Backend scheduler

ggml/src/ggml-backend-reg.cpp

@@ -637,3 +637,35 @@ void ggml_backend_load_all_variants(const char * name) {
         }
     }
 }
+
+void ggml_backend_load_variant(const char * name, const char * variant) {
+    const fs::path name_path = fs::u8path(name);
+    const fs::path variant_path = fs::u8path(variant);
+    const fs::path file_prefix = backend_filename_prefix().native() + name_path.native() + fs::u8path("-").native();
+    const fs::path target_filename = file_prefix.native() + variant_path.native() + backend_filename_extension().native();
+
+    std::vector<fs::path> search_paths;
+#ifdef GGML_BACKEND_DIR
+    search_paths.push_back(fs::u8path(GGML_BACKEND_DIR));
+#endif
+    // default search paths: executable directory, current directory
+    search_paths.push_back(get_executable_path());
+    search_paths.push_back(fs::current_path());
+
+    for (const auto & search_path : search_paths) {
+        if (!fs::exists(search_path)) {
+            GGML_LOG_DEBUG("%s: search path %s does not exist\n", __func__, path_str(search_path).c_str());
+            continue;
+        }
+
+        fs::path full_path = search_path / target_filename;
+        if (fs::exists(full_path) && fs::is_regular_file(full_path)) {
+            ggml_backend_reg_t backend = get_reg().load_backend(full_path, false);
+            if (backend == nullptr) {
+                GGML_LOG_ERROR("%s: failed to load backend variant %s\n", __func__, path_str(full_path).c_str());
+            } else {
+                return;
+            }
+        }
+    }
+}