fix argparse type error: replaced 'str' with str

[marccorretge]

This pull request fixes a ValueError caused by incorrectly using the string 'str' instead of the type str in the argparse argument definitions within opts.py.

Changes include:

• Correcting the type parameter in add_argument calls for --root_dataset, --path_AFLink, and --dir_save to use str instead of 'str'.

These changes resolve the error and ensure the script runs correctly. Please review and merge.

[dyhBUPT]

Thanks for your correcting! Looking forward to your more contributions~