Masking ray sampling

Hi,

Because my dataset has black regions in the images, I would like to ask how I can add my mask for ray sampling to avoid those black regions.

I checked the code and I think I should start from [idx_sample](https://github.com/facebookresearch/localrf/blob/main/localTensoRF/dataLoader/localrf_dataset.py#L333).
Do I have to be aware anywhere else?
Thank you.

Start at localTensoRF/dataLoader/localrf_dataset.py, especially idx_sample, and trace where rays are selected and consumed. Determine how the requested image mask should enter that path and verify that masked black regions are excluded during sampling; the issue does not identify tests or another entry point.

Written by the indexing model from the issue text.