Disables implicit GPTQ quantization using dtype_policy setter

keras/src/layers/layer.py

@@ -757,6 +757,15 @@ def dtype_policy(self, value):
             self._dtype_policy = policy
         if policy.quantization_mode is not None:
             if self.built and not getattr(self, "_is_quantized", False):
+                if policy.quantization_mode == "gptq":
+                    raise ValueError(
+                        "Implicitly enabling GPTQ quantization by setting "
+                        f"`dtype_policy` to '{value}' is not supported. "
+                        "GPTQ requires a calibration dataset and a "
+                        "`GPTQConfig` object.\n\n"
+                        "Please use the `.quantize('gptq', config=...)` method "
+                        "on the layer or model instead."
+                    )
                 self.quantize(policy.quantization_mode)
 
     @property

keras/src/layers/layer_test.py

@@ -233,6 +233,16 @@ def test_quantized_layer_with_remat(self):
         self.assertLen(mock_remat.rematted_functions, 1)
         next(iter(mock_remat.rematted_functions.values())).assert_called()
 
+    def test_gptq_quantization_by_setting_dtype(self):
+        """Tests error being raised when dtype is set to GPTQ."""
+        with self.assertRaisesRegex(
+            ValueError,
+            "Implicitly enabling GPTQ quantization.*is not supported",
+        ):
+            layer = layers.Dense(3)
+            layer.build((2, 4))
+            layer.dtype_policy = "gptq/4/-1_from_float32"
+
     def test_functional_model_with_remat(self):
         if backend.backend() in ("openvino", "numpy"):
             self.skipTest(