fix(quant): load offline-quantized compressed-tensors checkpoints directly on CPU under offload

The diffusion loader treated any quant config without the MXFP8/
is_checkpoint_quantized markers as online quantization: it loaded every
component on the accelerator before offloading, so serving the int8
weight-only compressed-tensors artifact OOM'd at model construction
(encoder 17 GiB + VAE + packed DiT skeleton > 24 GiB). compressed-tensors
is always a serialized format with no on-load quantization pass — detect
it (through component wrappers too) and load on the host, round-tripping
each quantized module through the accelerator only for the post-load
Marlin repack, which has no CPU path.

tests/quantization/test_quant_factory.py

@@ -12,6 +12,7 @@ from vllm.model_executor.layers.quantization.compressed_tensors.compressed_tenso
 from vllm_omni.quantization import build_quant_config, resolve_component_quant_config
 from vllm_omni.quantization.component_config import (
     ComponentQuantizationConfig,
+    is_prequantized_config,
     resolve_encoder_quant_config,
 )
 from vllm_omni.quantization.factory import SUPPORTED_QUANTIZATION_METHODS, _build_single
@@ -75,3 +76,18 @@ def test_unknown_method_still_raises() -> None:
 
 def test_supported_list_advertises_hyphenated_registry_names() -> None:
     assert "compressed-tensors" in SUPPORTED_QUANTIZATION_METHODS
+
+
+def test_is_prequantized_config_recognizes_serialized_formats() -> None:
+    # compressed-tensors is always a serialized format: its linear methods
+    # consume packed checkpoint tensors and there is no on-load quantization.
+    assert is_prequantized_config(build_quant_config(_int8_weight_only_disk_config()))
+    component = build_quant_config({"transformer": _int8_weight_only_disk_config()})
+    assert isinstance(component, ComponentQuantizationConfig)
+    assert is_prequantized_config(component)
+    assert not is_prequantized_config(None)
+    # Online (non-serialized) methods stay unrecognized so the loader keeps
+    # the accelerator-side load-quantize-offload path for them.
+    from vllm_omni.quantization.fp8_config import DiffusionFp8Config
+
+    assert not is_prequantized_config(DiffusionFp8Config())