Demo on video files is too slow

[HYLee1008]

Running demo_fit_body.py on video file is too slow. RCNN is running on cpu mode now. You need to change this line

https://github.com/YadiraF/PIXIE/blob/3162310d1507221aebca46721a0ec92d35d98cc4/demos/demo_fit_body.py#L31

to this

    testdata = TestData(args.inputpath, iscrop=args.iscrop, body_detector='rcnn', device=device)

[hshreeshail]

+1. Getting a ~2.25x speedup on a P40 GPU compared to CPU.