fix ELU output for nan, test=develop

Author: qili93

paddle/fluid/operators/activation_op.h

@@ -1141,9 +1141,9 @@ struct ELUFunctor : public BaseActivationFunctor<T> {
 
   template <typename Device, typename X, typename Out>
   void operator()(Device d, X x, Out out) const {
-    out.device(d) = x.cwiseMax(static_cast<T>(0)) +
-                    (static_cast<T>(alpha) * (x.exp() - static_cast<T>(1)))
-                        .cwiseMin(static_cast<T>(0));
+    out.device(d) =
+        (x < static_cast<T>(0))
+            .select(static_cast<T>(alpha) * (x.exp() - static_cast<T>(1)), x);
   }
 };