[Enhancement] Integrate Threestudio Support with LRM Initialization

[Adamdad]

[Enhancement] Integrate Threestudio Support with LRM Initialization for 3DGS

https://github.com/Adamdad/threestudio-lrm

Implemented an extension for threestudio using LRM (Linear Regression Model) as the initial setup in 3dgs for the purpose of score distillation. Kindly update the documentation to include this new functionality.