Add the AddCheckpoint macro to softplus op (#26809)

paddle/fluid/operators/activation_op.cc

@@ -1255,4 +1255,14 @@ REGISTER_OP_VERSION(hard_shrink)
                 "((x < -threshold) + (x > threshold)); after checkpoint: out = "
                 "x * (((x < -threshold) + (x > threshold)) > 0)"));
 
+REGISTER_OP_VERSION(softplus)
+    .AddCheckpoint(
+        R"ROC(add new attributes [beta] and [threshold], and the formula is changed to "
+         " softplus(x) = \\frac{1}{beta} * \\log(1 + e^{beta * x}) \\\\ \\text{For numerical"
+         " stability, the implementation reverts to the linear function when: beta * x > threshold.})ROC",
+        paddle::framework::compatible::OpVersionDesc()
+            .NewAttr("beta", "The beta value of the new formula", 1.0f)
+            .NewAttr("threshold", "The threshold value of the new formula",
+                     20.0f));
+
 /* ========================================================================== */