cuda : fallback to CPU on host buffer alloc fail

ggml-cuda.cu

@@ -9797,8 +9797,10 @@ static void ggml_backend_cuda_host_buffer_free_buffer(ggml_backend_buffer_t buff
 
 static ggml_backend_buffer_t ggml_backend_cuda_host_buffer_type_alloc_buffer(ggml_backend_buffer_type_t buft, size_t size) {
     void * ptr = ggml_cuda_host_malloc(size);
+
     if (ptr == nullptr) {
-        return nullptr;
+        // fallback to cpu buffer
+        return ggml_backend_buft_alloc_buffer(ggml_backend_cpu_buffer_type(), size);
     }
 
     // FIXME: this is a hack to avoid having to implement a new buffer type