Created by: kavyasrinet

Adding inference to the fluid fit_a_line model, this will help us when we re-write the models for the book. It will also help @kexinzhao and me understand how inference works, before we start working on inference wrapper for Mobile.

We have a few questions:

1. Is the way we perform inference in this example correct ?
2. How do we determine the program that gets run inside of exe.run during inference ? (Should we clone the fluid.default_main_program() before we start training or is the method I used here, correct ? )