Bodyposenet I want to start with usb camera video 0

You can try format with list: v4l2:///dev/video0 (Assume you have only one USB camera and the index is 0).
From the source code you can see he source-list is parsed by function nvds_parse_source_list and the source is created by function create_source_bin, which uses plugin uridecodebin and it has format v4l2:///* as above, refer the format FAQ here.