refactor detection predictor

[tuofeilunhifi]

Motivation

The use of unified DetectionPredictor for detection test and predictor.

For example:

1. for detection test

        model_path = '.pth'
        config_path = '.py'
        img = './demo.jpg'
        out_file = './result.jpg'
        detector = DetectionPredictor(model_path, config_path)
        output = detector.predict(img)
        detector.visualize(img, output, out_file=out_file)

2. for detection predictor

        model_path = '.pth'
        # config_path = '.py'   # remove config_path   
        img = './demo.jpg'
        out_file = './result.jpg'
        detector = DetectionPredictor(model_path)
        output = detector.predict(img)
        detector.visualize(img, output, out_file=out_file)