Do NOT expose temporary concepts like main_program and startup_program

[wangkuiyi]

Currently, in our official examples, like the following on

Paddle/python/paddle/fluid/tests/book/test_fit_a_line.py

Line 48 in 25317bd

def train_loop(main_program):

we exposed main program and startup program, which are concepts that shouldn't been there -- the only reason that we have them is because we didn't complete the implementation of the Fluid design.

Once we release a version in the current status, we are telling users that "you are recommended to create two kinds of programs explicitly" -- this is WRONG!

[wangkuiyi]

We should follow the design principle as explained in #7464