Created by: reyoung

The current imports of fluid is quite noisy. We could simplify them by adding imports in fluid/__init__.py https://github.com/PaddlePaddle/Paddle/blob/e800c0d3f1b53c839ecc1f4ec214ea572a45ed93/python/paddle/v2/fluid/tests/book/test_fit_a_line.py#L2-L8

The user API could be

import paddle.v2.fluid as fluid

data = fluid.layers.data(...)
loss = fluid.layers.fc(data, ...)
adam = fluid.optimizers.Adam()
adam.minimize(loss)
exe = fluid.Executor(...)
exe.run(feed={...}, fetch=[...])