Propose fit_a_line.py in the high-level (simplified) Fluid API

[wangkuiyi]

Inspired by @reyoung 's comment, I think we can base our discussions on the higher-level (or simplified) Fluid API on some solid examples. This also help us utilize the test-driven development (TDD) approach to make the new API happen.

[shanyi15]

您好，此issue在近一个月内暂无更新，我们将于今天内关闭。若在关闭后您仍需跟进提问，可重新开启此问题，我们将在24小时内回复您。因关闭带来的不便我们深表歉意，请您谅解~感谢您对PaddlePaddle的支持!
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