Show how to use the new visitors with lambdas (#2162)

examples/dnn_dcgan_train_ex.cpp

@@ -130,7 +130,9 @@ int main(int argc, char** argv) try
 
     // Instantiate both generator and discriminator
     generator_type generator;
-    discriminator_type discriminator(leaky_relu_(0.2), leaky_relu_(0.2), leaky_relu_(0.2));
+    discriminator_type discriminator;
+    // setup all leaky_relu_ layers in the discriminator to have alpha = 0.2
+    visit_computational_layers(discriminator, [](leaky_relu_& l){ l = leaky_relu_(0.2); });
     // Remove the bias learning from all bn_ inputs in both networks
     set_all_bn_inputs_no_bias(generator);
     set_all_bn_inputs_no_bias(discriminator);