Added forward pass for depth rendering

[JonathonLuiten]

Here are small changes required in this repo to align with the actual changes done in a PR in the renderer repo.

See here:
graphdeco-inria/diff-gaussian-rasterization#2

[grgkopanas]

Thank you, thats very useful.

We would need to check for regression in speed or add a guard that is by default False to accept this PR

[JonathonLuiten]

Hey @grgkopanas and @Snosixtyboo

I ran the speed test on this (tanks and temples / truck for 7000 iterations)

It results in 0 slowdown. I think it's safe to merge this.

Results:

WITH CHANGE:

Training progress: 23%|██▎ | 7000/30000 [02:50<12:04, 31.74it/s, Loss=0.0588340]
[ITER 7000] Evaluating train: L1 0.033260490745306015 PSNR 24.68514633178711 [13/07 15:25:22]

ORIGINAL:

Training progress: 23%|██▎ | 7000/30000 [02:50<12:07, 31.60it/s, Loss=0.0594036]
[ITER 7000] Evaluating train: L1 0.03345547989010811 PSNR 24.60102653503418 [13/07 15:32:42]