Created by: zhaoyuchen2018

• Improve topk performance.

give 200000 data to compute topk, before opt: cost 1s after opt: cost 0.0028s.

• Refine return value.
• Add cuda util funtions.
• Fix ComputeBlockSize bug & refine comments.

Signed-off-by: zhaoyuchen zhaoyuchen01@baidu.com