[Actorder] Fix GPTQ actorder serialization (#1818)

## Purpose ##
* Fix serialization of actorder sentinel values

## Prerequisites ##
* #1815

## Changes ##
* Write explicit serializer for actorder field
* Move down deprecated field checker, since it's less important to read
first

## Testing ##
* Added test which fails without these changes

---------

Signed-off-by: Kyle Sayers <kylesayrs@gmail.com>
Co-authored-by: Dipika Sikka <dipikasikka1@gmail.com>

Author: kylesayrs

src/llmcompressor/modifiers/quantization/gptq/base.py

@@ -120,17 +120,6 @@ class GPTQModifier(Modifier, QuantizationMixin):
     _hessians: Dict[torch.nn.Module, torch.Tensor] = PrivateAttr(default_factory=dict)
     _num_samples: Dict[torch.nn.Module, int] = PrivateAttr(default_factory=dict)
 
-    @field_validator("sequential_update", mode="before")
-    def validate_sequential_update(cls, value: bool) -> bool:
-        if not value:
-            warnings.warn(
-                "`sequential_update=False` is no longer supported, setting "
-                "sequential_update=True",
-                DeprecationWarning,
-            )
-
-        return True
-
     def resolve_quantization_config(self) -> QuantizationConfig:
         config = super().resolve_quantization_config()
 
@@ -317,3 +306,14 @@ def _maybe_onload_hessian(self, module: torch.nn.Module):
         if self.offload_hessians:
             if module in self._hessians:  # may have been deleted in context
                 self._hessians[module] = self._hessians[module].to(device="cpu")
+
+    @field_validator("sequential_update", mode="before")
+    def validate_sequential_update(cls, value: bool) -> bool:
+        if not value:
+            warnings.warn(
+                "`sequential_update=False` is no longer supported, setting "
+                "sequential_update=True",
+                DeprecationWarning,
+            )
+
+        return True

src/llmcompressor/sentinel.py

@@ -45,7 +45,13 @@ def __reduce__(self):
 
     @classmethod
     def __get_pydantic_core_schema__(cls, _source_type, _handler):
-        return core_schema.no_info_plain_validator_function(cls.validate)
+        return core_schema.no_info_after_validator_function(
+            cls.validate,
+            schema=core_schema.str_schema(),
+            serialization=core_schema.plain_serializer_function_ser_schema(
+                lambda v: str(v)
+            ),
+        )
 
     @classmethod
     def validate(cls, value: "Sentinel") -> "Sentinel":

tests/llmcompressor/modifiers/quantization/test_base.py

@@ -142,3 +142,21 @@ def test_config_resolution(strategies, actorder):
     for config_group in modifier.config_groups.values():
         if config_group.weights.strategy == "group":
             assert config_group.weights.actorder == actorder
+
+
+@pytest.mark.parametrize(
+    "has_actorder,actorder,exp_actorder",
+    [
+        (False, "N/A", "static"),
+        (True, None, None),
+        (True, "static", "static"),
+        (True, "group", "group"),
+    ],
+)
+def test_serialize_actorder(has_actorder, actorder, exp_actorder):
+    if has_actorder:
+        modifier = GPTQModifier(targets=["Linear"], actorder=actorder)
+    else:
+        modifier = GPTQModifier(targets=["Linear"])
+
+    assert modifier.model_dump()["actorder"] == exp_actorder