ggml : allow setting the backend name with env variable

ref: ggml-org#1058

Author: rgerganov

src/ggml-backend-reg.cpp

@@ -549,4 +549,9 @@ void ggml_backend_load_all_from_path(const char * dir_path) {
     ggml_backend_load_best("opencl", silent, dir_path);
     ggml_backend_load_best("musa", silent, dir_path);
     ggml_backend_load_best("cpu", silent, dir_path);
+    // check the environment variable GGML_BACKEND_NAME to load an out-of-tree backend
+    const char * backend_name = std::getenv("GGML_BACKEND_NAME");
+    if (backend_name) {
+        ggml_backend_load_best(backend_name, silent, dir_path);
+    }
 }