feat(pt/dpa4): add nvalchemi-toolkit warpper for DPA4

deepmd/pt/nvalchemi/__init__.py

@@ -0,0 +1,41 @@
+# SPDX-License-Identifier: LGPL-3.0-or-later
+"""Optional bridge between DeePMD-kit SeZM / DPA-4 models and NVIDIA's
+``nvalchemi-toolkit`` molecular-dynamics framework.
+
+``nvalchemi-toolkit`` is an optional dependency.  Importing this subpackage
+without it installed raises a clear, actionable error instead of an opaque
+``ModuleNotFoundError`` deep in the import chain.
+
+Example
+-------
+::
+
+    from deepmd.pt.nvalchemi import DPA4Wrapper
+    from nvalchemi.data import AtomicData, Batch
+    from nvalchemi.neighbors import compute_neighbors
+
+    model = DPA4Wrapper.from_checkpoint("model.pt", device="cuda")
+    batch = Batch.from_data_list([data], device="cuda")
+    compute_neighbors(batch, config=model.model_config.neighbor_config)
+    out = model(batch)  # {"energy": (B, 1), "forces": (N, 3), ...}
+"""
+
+from __future__ import (
+    annotations,
+)
+
+try:
+    import nvalchemi  # noqa: F401
+except ImportError as e:  # pragma: no cover - exercised only without the dep
+    raise ImportError(
+        "deepmd.pt.nvalchemi requires the optional `nvalchemi-toolkit` package. "
+        "Install it with `pip install deepmd-kit[nvalchemi]` "
+        "(or `pip install nvalchemi-toolkit`)."
+    ) from e
+
+from .dpa4wrapper import (
+    DPA4Wrapper,
+    SeZMWrapper,
+)
+
+__all__ = ["DPA4Wrapper", "SeZMWrapper"]