Add returns to plot utilities

[JulianMar11]

The plot utils in /mmcv/visualization/image.py do not have a return. Thus, if you want to plot GT boxes with PRED boxes in the same image or compare predictions of multiple models, the functions do not allow a pipeline-oriented interaction (such as torch transforms) with drawing utils.

[ZwwWayne]

Thanks for your suggestion. There are ongoing refactoring on the visualization API and we will take this into consideration.