MipNerf360 does not follow article and formal implementation of Prop Module

[doronpor]

MipNerf360 implementation diverges from the formal implementation and creates two different Prop modules. https://github.com/kakaobrain/nerf-factory/blob/main/src/model/mipnerf360/model.py#L229

In the article and formal implementation, a single Prop module is created and used (twice). https://github.com/google-research/multinerf/blob/main/internal/models.py#L99 https://github.com/google-research/multinerf/blob/main/internal/models.py#L221

as indicated by the article, in the beginning of 5. Optimization section: "We do two stages of evaluation and resampling of the proposal MLP"