Fixed: set default type for parser

[TelBotDev]

The value will be treated as str if there is no type. Then there will be error

Traceback (most recent call last):
  File "demo/visualize_result.py", line 144, in <module>
    query_indices = visualizer.vis_rank_list(args.output, args.vis_label, args.num_vis, args.rank_sort, args.label_sort, args.max_rank)
  File "./fastreid/utils/visualizer.py", line 158, in vis_rank_list
    query_indices = query_indices[:num_vis]
TypeError: slice indices must be integers or None or have an __index__ method

when we add some custom parameter, such as --num-vis 5