Created by: wangkuiyi

We'd thought that a DL framework should implement concepts like model and cost. But we realized that these are not flexible enough to describe deep learning problems. Instead, we need the concept network. For more about this derivation, please refer to https://github.com/PaddlePaddle/Paddle/issues/1311.

In this issue, we are going to figure out how should we build a network and its parameters, and how can we train a network and use part of it (the model) for inference/serving.