Created by: nickyfantasy

According to this PR: https://github.com/guoshengCS/Paddle/blob/d15b2e02c8a1fbfdac4b8a1ee6b7f9809eb0abdd/python/paddle/fluid/tests/book/high-level-api/machine_translation/test_machine_translation.py

beam_search api has been updated, need to refract in book example