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>

train.py

@@ -463,7 +463,7 @@ def train(hyp,  # path/to/hyp.yaml or hyp dictionary
     return results
 
 
-if __name__ == '__main__':
+def parse_opt():
     parser = argparse.ArgumentParser()
     parser.add_argument('--weights', type=str, default='yolov5s.pt', help='initial weights path')
     parser.add_argument('--cfg', type=str, default='', help='model.yaml path')
@@ -504,6 +504,11 @@ if __name__ == '__main__':
     # Set DDP variables
     opt.world_size = int(getattr(os.environ, 'WORLD_SIZE', 1))
     opt.global_rank = int(getattr(os.environ, 'RANK', -1))
+    return opt
+
+
+def main(opt):
+    print(opt)
     set_logging(opt.global_rank)
     if opt.global_rank in [-1, 0]:
         check_git_status()
@@ -628,3 +633,8 @@ if __name__ == '__main__':
         plot_evolution(yaml_file)
         print(f'Hyperparameter evolution complete. Best results saved as: {yaml_file}\n'
               f'Command to train a new model with these hyperparameters: $ python train.py --hyp {yaml_file}')
+
+
+if __name__ == "__main__":
+    opt = parse_opt()
+    main(opt)