Need `.param` interface for the Variable that returned by layer function
Created by: reyoung
It is common to get the parameters of layers. However, it is not supported by fluid now.
It can be implemented by adding .param for Variables that returned by layers.
Example usages are:
hidden = fc(input=[a, b], ...)
prediction = fc(input=hidden, ...)
...
exe.run(fetch=[hidden.param[0], hidden.param[1], hidden.bias, prediction.param])