Do NOT expose the concept Executor

[wangkuiyi]

Fluid was designed a trail towards a new DL programming language. It doesn't make sense for users to write a Python program that calls the Python interpreter. However, this is what we are doing right now:

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

Line 104 in 25317bd

exe = fluid.Executor(place)

Let us remove the expose of Executor, and follow the design #7464.

[wangkuiyi]

The correct solution is to use the standard package atexit:

• When we import paddle.fluid, we run the initialization module of Fluid, which should call atexit.register(run_fluid_program).
• When the Python interpret runs the Fluid application program, it generates a ProgramDesc message.
• When the application completes, Python calls run_fluid_program, which should call the C++ class Executor run interpret and run the generated ProgramDesc message.