distortion loss impl.

[liruilong940607]

Implement distortion loss for 3DGS from Mip-NeRF 360:

Note: Since the distortion loss is a per-pixel value, I implement it as a per-pixel map rendered from the rasterization() function, with the same resolution with the rendered image. This could be reduced into a scalar with a .mean() call on it for backward. Having the distortion loss as a map instead of a scalar could be useful for visualization and analysis purposes.