fix(pt_expt): freeze a graph-capable model through the graph lower

dp freeze resolved the lower kind with an isinstance(NativeSpinModelKind)
special case, so native-spin models were auto-upgraded to the graph lower
but nothing else was. A ZBL-bridged model is not an instance of that type,
yet it has no dense route either -- the analytical term's forward_atomic
raises there -- so the public default lower_kind="nlist" failed deep
inside the dense trace instead of freezing.

Ask the graph-lower capability instead: the dense lower is deprecated in
this backend, so any model that can ride the graph lower now does. That
covers native spin, bridging, and the plain graph-capable descriptors in
one rule, with no wrapper-type test.

Regression: a bridged checkpoint frozen through the public freeze() with
no lower_kind and a suffixless output must yield a graph-kind .pt2, next
to the existing native-spin twin.

Author: Han Wang

deepmd/pt_expt/entrypoints/main.py

@@ -600,19 +600,21 @@ def freeze(
 
     m.eval()
 
-    # Native-spin models implement ONLY the NeighborGraph lower (no
-    # dense/nlist lower exists), so resolve the lower kind HERE -- before the
-    # export ABI is chosen and before the default output suffix below is
-    # derived from it. Without this, the public default (lower_kind="nlist")
-    # would reach the dense-spin trace branch and fail deep inside tracing.
-    from deepmd.dpmodel.model.native_spin_model import (
-        NativeSpinModelKind,
+    # A graph-capable model ALWAYS freezes through the NeighborGraph lower:
+    # the dense (nlist) lower is deprecated in this backend, and for some
+    # architectures it does not exist at all (native spin has no dense spin
+    # lower; an analytical bridging term has no dense injection site), so the
+    # public default ``lower_kind="nlist"`` would fail deep inside the dense
+    # trace. Resolved HERE -- before the export ABI is chosen and before the
+    # default output suffix below is derived from it.
+    from deepmd.pt_expt.model.graph_lower import (
+        model_uses_graph_lower,
     )
 
-    if isinstance(m, NativeSpinModelKind) and lower_kind != "graph":
+    if lower_kind != "graph" and model_uses_graph_lower(m):
         log.info(
-            "Native-spin model implements only the NeighborGraph lower; "
-            "resolving lower_kind=%r -> 'graph'.",
+            "Model is graph-lower capable; resolving lower_kind=%r -> 'graph' "
+            "(the dense lower is deprecated in the pt_expt backend).",
             lower_kind,
         )
         lower_kind = "graph"

source/tests/pt_expt/model/test_dpa4_export.py

@@ -520,6 +520,48 @@ def test_native_spin_default_freeze_routes_to_graph(tmp_path) -> None:
     assert md["is_spin"] is True
 
 
+def test_bridged_default_freeze_routes_to_graph(tmp_path) -> None:
+    """A ZBL-bridged model also default-freezes to a graph-kind ``.pt2``.
+
+    The dense lower is deprecated in this backend, and for a bridged model
+    it does not exist at all: the analytical term's ``forward_atomic``
+    raises on the dense route, so the public default
+    ``lower_kind="nlist"`` used to fail deep inside the dense trace. The
+    resolution was previously an ``isinstance(NativeSpinModelKind)``
+    special case, which a bridged model is not an instance of; it now asks
+    the graph-lower capability, which covers both.
+    """
+    import copy
+
+    from deepmd.pt_expt.entrypoints.main import (
+        freeze,
+    )
+    from deepmd.pt_expt.model.get_model import (
+        get_model,
+    )
+    from deepmd.pt_expt.train.wrapper import (
+        ModelWrapper,
+    )
+
+    config = copy.deepcopy(_DPA4_CONFIG)
+    config["bridging_method"] = "ZBL"
+    config["bridging_r_inner"] = 0.8
+    config["bridging_r_outer"] = 1.2
+    model = get_model(copy.deepcopy(config)).to(torch.device("cpu")).eval()
+    wrapper = ModelWrapper(model, model_params=copy.deepcopy(config))
+    ckpt = tmp_path / "model.pt"
+    torch.save({"model": wrapper.state_dict()}, ckpt)
+
+    output = tmp_path / "frozen_bridged"  # suffixless: default CLI form
+    freeze(model=str(ckpt), output=str(output))
+
+    pt2 = output.with_suffix(".pt2")
+    assert pt2.exists(), "default suffix must follow the resolved graph kind"
+    with zipfile.ZipFile(pt2) as z:
+        md = json.loads(z.read("model/extra/metadata.json").decode("utf-8"))
+    assert md["lower_input_kind"] == "graph"
+
+
 def test_virtual_spin_graph_freeze_still_rejected(tmp_path) -> None:
     """spin_ener (virtual) graph freeze keeps raising ``NotImplementedError``.