Viewmat gradient backward to optimize camera position

[pierremerriaux-leddartech]

Hi @vye16 and @kerrj In order to re activate camera pose opt in nerfstudio, I cherry pick the gradient backward proposed in branch : https://github.com/nerfstudio-project/gsplat/tree/vickie/camera-grads

My tests were not really good. Did you get any plan to continue with this feature ? Anyway I could help on it ? Any idea how we could test properly. thanks