The activation of img_conv_layer is not consistent in codes and doc.
Created by: lcy-seso
The activation of img_conv_layer is set to Relu in codes (https://github.com/PaddlePaddle/Paddle/blob/develop/python/paddle/trainer_config_helpers/layers.py#L2025),
while in doc, it is said to be "Tanh" (https://github.com/PaddlePaddle/Paddle/blob/develop/python/paddle/trainer_config_helpers/layers.py#L2093).