Slightly modify CLI execution (#3687)

* Slightly modify CLI execution

This simple change makes it easier to run the primary functions of this
repo (train/detect/test) from within Python. An object which represents
`opt` can be constructed and fed to the `main` function of each of these
modules, rather than having to call the lower level functions directly,
or run the module as a script.

* Update export.py

Add CLI parsing update for more convenient module usage within Python.

Co-authored-by: Lewis Belcher <lb@desupervised.io>

detect.py

@@ -172,7 +172,7 @@ def detect(weights='yolov5s.pt',  # model.pt path(s)
     print(f'Done. ({time.time() - t0:.3f}s)')
 
 
-if __name__ == '__main__':
+def parse_opt():
     parser = argparse.ArgumentParser()
     parser.add_argument('--weights', nargs='+', type=str, default='yolov5s.pt', help='model.pt path(s)')
     parser.add_argument('--source', type=str, default='data/images', help='file/dir/URL/glob, 0 for webcam')
@@ -198,7 +198,15 @@ if __name__ == '__main__':
     parser.add_argument('--hide-conf', default=False, action='store_true', help='hide confidences')
     parser.add_argument('--half', action='store_true', help='use FP16 half-precision inference')
     opt = parser.parse_args()
+    return opt
+
+
+def main(opt):
     print(opt)
     check_requirements(exclude=('tensorboard', 'thop'))
-
     detect(**vars(opt))
+
+
+if __name__ == "__main__":
+    opt = parse_opt()
+    main(opt)