[Quantization] Fix GPTQ scheme attachment broken by LinearBase.scheme default (#34962)

Co-authored-by: Mohammad Angkad <mohammad.angkad@radixark.ai>

test/registered/unit/layers/quantization/test_gptq_scheme_attach.py

@@ -0,0 +1,60 @@
+"""GPTQ builds its per-layer scheme lazily in `create_weights`, so the layer has
+to declare `scheme = None` for the `is None` probe to see it.
+
+Regression: the probe used to be `hasattr(layer, "scheme")`, which degraded to
+always-true once `LinearBase` grew that class default -- the scheme was never
+built and every GPTQ model died with ``'NoneType' object has no attribute
+'create_weights'``.
+"""
+
+import unittest
+
+import torch
+
+from sglang.srt.layers.linear import LinearBase, ReplicatedLinear
+from sglang.srt.layers.moe.fused_moe_triton import FusedMoE
+from sglang.srt.layers.quantization.gptq.gptq import GPTQConfig
+from sglang.srt.layers.vocab_parallel_embedding import VocabParallelEmbedding
+from sglang.test.ci.ci_register import register_cpu_ci
+from sglang.test.test_utils import CustomTestCase
+
+register_cpu_ci(est_time=5, suite="base-a-test-cpu")
+
+_GPTQ_CHECKPOINT_CONFIG = {
+    "bits": 4,
+    "group_size": 128,
+    "desc_act": False,
+    "lm_head": False,
+    "dynamic": {},
+    "checkpoint_format": "gptq",
+    "true_sequential": True,
+    "static_groups": False,
+}
+
+
+class TestGPTQSchemeAttach(CustomTestCase):
+    def test_linear_layer_gets_a_scheme(self):
+        layer = ReplicatedLinear(
+            input_size=256,
+            output_size=128,
+            bias=False,
+            params_dtype=torch.float16,
+            quant_config=GPTQConfig.from_config(_GPTQ_CHECKPOINT_CONFIG),
+            prefix="model.layers.0.mlp.down_proj",
+        )
+        self.assertIsNotNone(layer.scheme)
+        self.assertTrue(hasattr(layer, "qweight"))
+
+    def test_scheme_default_is_declared_on_every_quantizable_layer_base(self):
+        """`get_linear_quant_method` hands a linear method a `LinearBase` or a
+        quantized `ParallelLMHead`; `GPTQMarlinConfig` hands
+        `GPTQMarlinMoEMethod` a bare `FusedMoE`. The MoE attach has no e2e
+        coverage, so this is its only guard.
+        """
+        self.assertIsNone(LinearBase.scheme)
+        self.assertIsNone(VocabParallelEmbedding.scheme)
+        self.assertIsNone(FusedMoE.scheme)
+
+
+if __name__ == "__main__":
+    unittest.main()