Update parse_opt() to work as in train.py

Change parse_opt() be able to use parse_known_args(), same as in train.py, so export.main() can be called from other script without error.

e.g.:

    from yolov5 import export
    opt = export.parse_opt(True)
    opt.weights = <model_path>
    opt.include = ("torchscript", "onnx")
    opt.data = <data>
    opt.imgsz = [<height>, <width>]
    export.main(opt)
Signed-off-by: Johan Bergman <35481994+duran67@users.noreply.github.com>
Signed-off-by: Johan Bergman <35481994+duran67@users.noreply.github.com>