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>

models/export.py

@@ -144,7 +144,7 @@ def export(weights='./yolov5s.pt',  # weights path
     print(f'\nExport complete ({time.time() - t:.2f}s). Visualize with https://github.com/lutzroeder/netron.')
 
 
-if __name__ == '__main__':
+def parse_opt():
     parser = argparse.ArgumentParser()
     parser.add_argument('--weights', type=str, default='./yolov5s.pt', help='weights path')
     parser.add_argument('--img-size', nargs='+', type=int, default=[640, 640], help='image (height, width)')
@@ -159,7 +159,15 @@ if __name__ == '__main__':
     parser.add_argument('--simplify', action='store_true', help='ONNX: simplify model')
     parser.add_argument('--opset-version', type=int, default=12, help='ONNX: opset version')
     opt = parser.parse_args()
+    return opt
+
+
+def main(opt):
     print(opt)
     set_logging()
-
     export(**vars(opt))
+
+
+if __name__ == "__main__":
+    opt = parse_opt()
+    main(opt)