Created by: wangkuiyi

Fluid layers are defined in the Python package layers, which implies that when users are programming Fluid application, they'd have to write

y = fluid.layers.fc(x, ...)

However, the right way is layer.fc, instead of layers.fc:

y = fluid.layer.fc(x, ...)

I reminded this issue months ago, but we still have not corrected it. The later we do so, the more application code we'd have to correct accordingly. So, let us do it right now!