num_beams for beam search

[shammmmmless]

Hi, I want to use num_beams for generate, but PipelineModel can't. Can you support num_beams?

Best wishes.

[dujiangsu]

Since we are working on imporving a large-scale model demo recently, the requirement might be delayed for one or two weeks. You can use beam search in transformer and add it to your code following this. Also, welcome you to contribute your beam code usage to the project.