Fix imread call to use apply_gamma=False instead of ignoregamma=True

[JY251]

Refactor imread function call to use apply_gamma=False for compatibility with latest imageio

In load_llff.py, updated the imread function call to use apply_gamma=False instead of the deprecated ignoregamma=True argument. This change ensures compatibility with the latest version of imageio, addressing issues reported in https://github.com/bmild/nerf/issues/190.