fix elu grad whne alpha less then zero, test=develop (#26543)

paddle/fluid/operators/activation_op.h

@@ -1134,9 +1134,20 @@ struct ELUGradFunctor : public BaseActivationFunctor<T> {
   template <typename Device, typename X, typename Out, typename dOut,
             typename dX>
   void operator()(Device d, X x, Out out, dOut dout, dX dx) const {
-    dx.device(d) = dout * (x > static_cast<T>(0)).template cast<T>() +
-                   dout * static_cast<T>(alpha) * x.exp() *
-                       (x <= static_cast<T>(0)).template cast<T>();
+    auto temp_a_pos = static_cast<T>(alpha > 0);
+    auto temp_a_neg = static_cast<T>(alpha <= 0);
+    auto temp_x_pos = (x > static_cast<T>(0)).template cast<T>();
+    auto temp_x_neg = (x <= static_cast<T>(0)).template cast<T>();
+
+    // dx = dout, if alpha > 0 and x > 0
+    // dx = dout * alpha * x.exp(), if alpha > 0 and x <= 0
+    // dx = dout * (1 + alpha * x.exp()), if alpha <= 0 and x > 0
+    // dx = 0, if alpha <= 0 and x <=0
+    dx.device(d) =
+        dout * temp_a_pos * temp_x_pos +
+        dout * static_cast<T>(alpha) * x.exp() * temp_a_pos * temp_x_neg +
+        dout * (static_cast<T>(1) + static_cast<T>(alpha) * x.exp()) *
+            temp_a_neg * temp_x_pos;
   }
 
   static constexpr ActBwdOpFwdDeps FwdDeps() { return kDepX; }