refactor: extract shared tabulate math, remove pt_expt→pt dependency

deepmd/dpmodel/utils/network.py

@@ -44,6 +44,18 @@ def sigmoid_t(x):  # noqa: ANN001, ANN201
     return xp_sigmoid(x)
 
 
+def softplus_t(x):  # noqa: ANN001, ANN201
+    """Numerically stable softplus."""
+    xp = array_api_compat.array_namespace(x)
+    positive = x > 0
+    exp_neg_abs = xp.exp(xp.where(positive, -x, x))
+    return xp.where(
+        positive,
+        x + xp.log(1 + exp_neg_abs),
+        xp.log(1 + exp_neg_abs),
+    )
+
+
 class Identity(NativeOP):
     def __init__(self) -> None:
         super().__init__()
@@ -330,9 +342,7 @@ def fn(x):  # noqa: ANN001, ANN202
     elif activation_function == "softplus":
 
         def fn(x):  # noqa: ANN001, ANN202
-            xp = array_api_compat.array_namespace(x)
-            # generated by GitHub Copilot
-            return xp.log(1 + xp.exp(x))
+            return softplus_t(x)
 
         return fn
     elif activation_function == "sigmoid":