Do not use list literal in Python as parameter default value
Created by: reyoung
Default parameter value is mutable in Python. The following lines are buggy. https://github.com/PaddlePaddle/Paddle/blob/74ead9dceab728a6ab6b642e6d37ba4406d4a1e3/python/paddle/v2/fluid/layers.py#L878-L879