[Diffusion] Make no-AllGather host weight cache automatic

Signed-off-by: Hongsheng Liu <liuhongsheng4@huawei.com>

tests/diffusion/model_loader/test_diffusers_loader.py

@@ -375,8 +375,9 @@ def test_dlo_plan_loads_component_sources_outside_planned_dit(monkeypatch):
     assert loader.take_host_weight_plan() is plan
 
 
-def test_dlo_plan_fallback_runs_ordinary_loader(monkeypatch):
+def test_dlo_plan_fallback_runs_ordinary_loader_then_cache(monkeypatch):
     import vllm_omni.diffusion.model_loader.diffusers_loader as loader_mod
+    import vllm_omni.diffusion.model_loader.host_weight_cache as cache_mod
 
     od_config = SimpleNamespace(
         dtype=torch.float32,
@@ -401,8 +402,14 @@ def test_dlo_plan_fallback_runs_ordinary_loader(monkeypatch):
         lambda *_args, **_kwargs: HostWeightPlanResult(None, "not direct-compatible"),
     )
 
+    def build_cache(_pipeline, **_kwargs):
+        calls.append("cache")
+        return HostWeightPlanResult(None, "unsupported test layout", "unsupported_layout")
+
+    monkeypatch.setattr(cache_mod, "build_host_weight_cache_plan", build_cache)
+
     assert loader.load_model(load_device="cpu") is model
-    assert calls == ["load", "process"]
+    assert calls == ["load", "process", "cache"]
     assert loader.take_host_weight_plan() is None
 
 
@@ -427,7 +434,6 @@ def test_dlo_host_weight_cache_is_built_after_final_post_load_mutation(monkeypat
         quantization_config=None,
         enable_distributed_layerwise_offload=True,
         dlo_use_allgather=False,
-        dlo_enable_host_weight_cache=True,
         dlo_host_weight_cache_dir=str(tmp_path),
         dlo_host_weight_cache_lock_timeout=1.0,
         model="unused",
@@ -509,7 +515,6 @@ def test_dlo_registration_prefers_final_host_weight_cache_over_checkpoint_plan(m
         quantization_config=None,
         enable_distributed_layerwise_offload=True,
         dlo_use_allgather=False,
-        dlo_enable_host_weight_cache=True,
         dlo_host_weight_cache_dir=str(tmp_path),
         dlo_host_weight_cache_pin_limit_gib=1.0,
         model="unused",
@@ -562,7 +567,6 @@ def test_dlo_host_weight_cache_failure_retains_ordinary_weights(monkeypatch, tmp
         quantization_config=None,
         enable_distributed_layerwise_offload=True,
         dlo_use_allgather=False,
-        dlo_enable_host_weight_cache=True,
         dlo_host_weight_cache_dir=str(tmp_path),
         model="unused",
     )