Guard descriptor extraction grad requirement

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: zhaiwenxi

deepmd/dpa_tools/_backend.py

@@ -158,6 +158,8 @@ def _run_forward(self, coord, atype, box):
         torch.Tensor
             (n_frames, n_atoms, feat_dim), detached.
         """
+        if not coord.requires_grad:
+            raise RuntimeError("forward_common requires coord to have requires_grad=True")
         self._clear_accumulator()
         self._inner_model.forward_common(coord, atype, box)
         return self._atomic_model.eval_descriptor().detach()