Add blender dataset + alpha training support

[thomakah]

This pull request adds the following:

• Support for the commonly-used synthetic blender dataset
• Support for source images with alpha channels, and small changes to training and evaluation to accommodate this (more info below)

The latter is the main intended purpose of the pull request, and the former is to allow for convenient testing on a public dataset with transparent images. Building off of this work, users can train models using matted photos to only produce splats for the object of interest - however this would require a small additional change on the COLMAP parsing to load the alpha channel (and is not included in this pull request).

Alpha is handled in the following way:

• For evaluation, both the render and ground truth photo are composed on a fixed background (defined by a new configuration parameters, defaulting to white).
• For training
  • if the (existing but slightly repurposed) configuration setting random_bkgd is True: on each iteration the same random background color is applied for both the photo and render
  • otherwise: the fixed background color is used

Using the random background in training encourages better alpha consistency and reduces floaters. However, the final evaluation metrics are marginally worse than if using a fixed background. For this reason, I have not enforced a setting of random_bkgd = True when blender data is used.

The metrics are included below. alpha_iou is the intersection over union comparing the rendered alpha to the source image alpha channel (thresholding on > 127). nerfbaselines has also assessed the Blender dataset on an earlier version of gsplat (patching in the dataset compatibility on their end). Compared to their results, 6 scenes show marginally better PSNR while 2 (materials and ficus) have marginally worse PSNR.

scene	strategy	background	psnr	ssim	lpips	alpha_iou
chair	default	fixed_bkgd	35.823	0.987	0.008	0.991
chair	default	random_bkgd	35.718	0.987	0.008	0.997
chair	mcmc	fixed_bkgd	36.398	0.989	0.008	0.993
chair	mcmc	random_bkgd	36.149	0.989	0.009	0.997
drums	default	fixed_bkgd	26.121	0.954	0.039	0.982
drums	default	random_bkgd	26.024	0.953	0.040	0.989
drums	mcmc	fixed_bkgd	26.209	0.956	0.040	0.973
drums	mcmc	random_bkgd	26.127	0.955	0.042	0.989
ficus	default	fixed_bkgd	34.360	0.987	0.010	0.952
ficus	default	random_bkgd	33.639	0.987	0.012	0.965
ficus	mcmc	fixed_bkgd	34.957	0.988	0.008	0.958
ficus	mcmc	random_bkgd	34.023	0.987	0.011	0.967
hotdog	default	fixed_bkgd	37.668	0.985	0.014	0.981
hotdog	default	random_bkgd	37.417	0.985	0.013	0.998
hotdog	mcmc	fixed_bkgd	38.373	0.988	0.011	0.762
hotdog	mcmc	random_bkgd	37.717	0.988	0.010	0.997
lego	default	fixed_bkgd	35.286	0.981	0.012	0.996
lego	default	random_bkgd	35.046	0.981	0.013	0.997
lego	mcmc	fixed_bkgd	35.732	0.984	0.011	0.994
lego	mcmc	random_bkgd	35.431	0.984	0.012	0.997
materials	default	fixed_bkgd	29.885	0.960	0.019	0.997
materials	default	random_bkgd	29.791	0.960	0.021	0.997
materials	mcmc	fixed_bkgd	30.577	0.965	0.018	0.997
materials	mcmc	random_bkgd	30.464	0.965	0.018	0.997
mic	default	fixed_bkgd	35.335	0.991	0.006	0.992
mic	default	random_bkgd	35.256	0.992	0.006	0.994
mic	mcmc	fixed_bkgd	37.226	0.994	0.004	0.992
mic	mcmc	random_bkgd	36.522	0.993	0.005	0.994
ship	default	fixed_bkgd	30.764	0.906	0.088	0.955
ship	default	random_bkgd	30.324	0.906	0.088	0.994
ship	mcmc	fixed_bkgd	31.437	0.911	0.075	0.829
ship	mcmc	random_bkgd	30.739	0.910	0.077	0.994

[thomakah]

To better illustrate the benefit of the random background, here is a validation render from training with a fixed background (using MCMC, though we see similar effects with the default strategy). The figures show the ground truth image on the left, and the corresponding render on the right.

Here is the validation render using a random background:

Note that the large patch of white floaters below the object is no longer present after using the random background in training.