--val_img_txt required to run main.py

[asarnow]

The docs say validation files are optional, but there is a file not found error for data/val_images.txt if refinement with main.py is run without --val_img_txt. Eventually there will also be an error for the default validation coordinate file name as well.

[huangqinwen]

Thanks for pointing out! We fixed this error and now the default for val_img_txt and val_coord_txt is empty. If there's no input to these arguments val_img_txt will be the same as train_img_txt