feat: support backend-specific max-vram budgets

examples/common/common.h

@@ -144,7 +144,7 @@ struct SDContextParams {
     rng_type_t rng_type         = CUDA_RNG;
     rng_type_t sampler_rng_type = RNG_TYPE_COUNT;
     bool offload_params_to_cpu  = false;
-    float max_vram              = 0.f;
+    std::string max_vram        = "0";
     bool stream_layers          = false;
     std::string backend;
     std::string params_backend;