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>

test.py

@@ -294,7 +294,7 @@ def test(data,
     return (mp, mr, map50, map, *(loss.cpu() / len(dataloader)).tolist()), maps, t
 
 
-if __name__ == '__main__':
+def parse_opt():
     parser = argparse.ArgumentParser(prog='test.py')
     parser.add_argument('--data', type=str, default='data/coco128.yaml', help='dataset.yaml path')
     parser.add_argument('--weights', nargs='+', type=str, default='yolov5s.pt', help='model.pt path(s)')
@@ -319,6 +319,10 @@ if __name__ == '__main__':
     opt.save_json |= opt.data.endswith('coco.yaml')
     opt.save_txt |= opt.save_hybrid
     opt.data = check_file(opt.data)  # check file
+    return opt
+
+
+def main(opt):
     print(opt)
     check_requirements(exclude=('tensorboard', 'thop'))
 
@@ -344,3 +348,8 @@ if __name__ == '__main__':
             np.savetxt(f, y, fmt='%10.4g')  # save
         os.system('zip -r study.zip study_*.txt')
         plot_study_txt(x=x)  # plot
+
+
+if __name__ == "__main__":
+    opt = parse_opt()
+    main(opt)