Kit Visualizer: Don't author omni:scenePartition attribute onto viewport camera when visualizing all envs

[rilei-nvidia]

Description

Previously, the value "env_0" was authored onto the Perspective Camera when using the kit visualizer. E.g.

./isaaclab.sh train --rl_library rsl_rl \
    --task Isaac-Lift-KukaAllegro-Camera \
    presets=newton_mjwarp,isaacsim_rtx_renderer,rgb64,single_camera \
    --seed 42 \
    --num_envs 4 \
    --max_iterations 100 \
    --visualizer kit

This caused only the first environment to be made visible, per scenePartition behavior:

In the case where we want to visualize all envs (_resolved_visible_env_ids is None) - we should not be authoring the omni:scenePartition attribute at all.

The reason this is a draft PR is that there is a bug in IsaacSim RTX renderer where if a camera does not have the omni:scenePartition attribute, none of the partitioned prims are visible to the viewport (below I've removed the attribute):

Checklist

• I have read and understood the contribution guidelines
• I have run the pre-commit checks with ./isaaclab.sh --format
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• [] I have added tests that prove my fix is effective or that my feature works
• I have updated the changelog and the corresponding version in the extension's config/extension.toml file
• I have added my name to the CONTRIBUTORS.md or my name already exists there