Pixel Kernels (3/2): RGBD Kernels

[horizon-blue]

While working on the image kernels, I realized that it might be useful to have a unified class that sample the RGBD values jointly, so I'm introducing this utility class that takes in an color kernel (from #147) and a depth kernel (from #149) to create an RGBD kernel, which supports sample and logpdf computation.

This is a general class that's agnostic of the type of RGB and depth kernels -- it'll pass in the arguments it receives to both of the kernels during sample and logpdf call. Since both color and depth kernels have *args, **kwargs as part of their function signature, they should simply ignore additional arguments that's not relevant to them.

(I also just realized that I was referring to these classes as "kernels" but the actual class names are *Distributions... maybe I should fix these in a future PR.)

I'm submitting this PR to main for now. Once I'm wrapping up my local changes, I'm going to work on resolving merge conflicts with gen3d.

Test Plan

Similar to the previous PRs, I've added some unit tests to make sure that the kernels roughly have the behaviors that we expected:

pytest tests/gen3d/test_pixel_rgbd_kernels.py