Update FP16 --half argument for test.py and detect.py (#3532)

* Update FP16 `--half` argument for test.py and detect.py

* Update detect.py

detect.py

@@ -172,7 +172,7 @@ if __name__ == '__main__':
     parser.add_argument('--line-thickness', default=3, type=int, help='bounding box thickness (pixels)')
     parser.add_argument('--hide-labels', default=False, action='store_true', help='hide labels')
     parser.add_argument('--hide-conf', default=False, action='store_true', help='hide confidences')
-    parser.add_argument('--half', type=bool, default=False, help='use FP16 half-precision inference')
+    parser.add_argument('--half', action='store_true', help='use FP16 half-precision inference')
     opt = parser.parse_args()
     print(opt)
     check_requirements(exclude=('tensorboard', 'pycocotools', 'thop'))