Added All pixel Sampler

[IamMohitM]

A sampler that selects a specific image and samples all pixels from that image.

I believe this could be a useful class where the neural field is optimised per image. This is used in nerf2mesh, where the pixels must be sampled from a single image to generate rays, which are then used to rasterise the image from the coarse mesh.

I use this pixel sampler in my implementation of nerf2mesh using nerfstudio.

Happy to make any edits if required.

[jb-ye]

Just some side question: I am also curious why we didn't have something like AllPixelSampler for implementing next_eval_image.

I spent sometime digging into the code and realized that we currently use RandIndicesEvalDataloader to select a random camera and use get_outputs_for_camera which involves a nested call of camera.generate_rays.

The process to obtain rays is not following the typical workflow we set for next_train() and next_eval(), where a pixel sampler is used.

[IamMohitM]

That makes sense. I didn't notice the RandIndicesEvalDataloader earlier.

The AllPixelSampler implementation makes it compatible with the next_train() and next_eval() function in the VanillaDataManager class. Both functions assert a requirement for a pixel sampler.