use RendererCfg as default renderer_cfg in CameraCfg

[r-schmitt]

Description

the camera config was importing isaaclab_physx.renderers because the default render_cfg was set to that config. this PR sets that to RendererConfig to remove the import, but provides a get_default_render_config method to the backend_utils to lazily import the config if needed. this is called post_init on the camera config to replace the generic config as soon as possible to avoid downstream issues referencing the renderer config. this action can be moved to the factory if downstream references are cleaned up.

Type of change

• Refactor to remove imports in cfg class

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

[greptile-apps]

Greptile Summary

This PR removes the eager top-level import of isaaclab_physx.renderers.IsaacRtxRendererCfg from camera_cfg.py by defaulting renderer_cfg to None and resolving it lazily in __post_init__ via the new get_default_render_cfg() helper in backend_utils.py.

• camera_cfg.py: renderer_cfg type widens to RendererCfg | None; __post_init__ calls get_default_render_cfg() to fill it in immediately, preserving existing downstream behaviour.
• backend_utils.py: Adds get_default_render_cfg(), which uses importlib.import_module with distinct error handling for a missing module vs. a missing class attribute.
• changelog.d/rschmitt_default_cameracfg_renderer.rst: Both :meth: cross-references spell the function name as get_default_renderer_cfg (with an extra renderer) instead of the actual get_default_render_cfg, producing broken Sphinx links.

Confidence Score: 4/5

Safe to merge; the lazy-import refactor works correctly at runtime and the post_init guard is solid. The only issue is a misspelled function name in the changelog RST that will produce broken Sphinx cross-references.

The core logic change — deferring the isaaclab_physx import to post_init — is well-structured and does not alter runtime behaviour for existing users. The sole finding is a mismatched function name in the changelog's :meth: references (get_default_renderer_cfg vs get_default_render_cfg), which breaks Sphinx link resolution but has no impact on the running code.

source/isaaclab/changelog.d/rschmitt_default_cameracfg_renderer.rst — both :meth: references use the wrong function name.

Important Files Changed

Filename	Overview
source/isaaclab/isaaclab/sensors/camera/camera_cfg.py	Changes renderer_cfg default from IsaacRtxRendererCfg() to None, with post_init lazily resolving it via get_default_render_cfg(); logic is sound and type annotation updated accordingly.
source/isaaclab/isaaclab/utils/backend_utils.py	Adds get_default_render_cfg() with proper lazy-import pattern, distinct error handling for missing module vs missing class, and TYPE_CHECKING guard for the return type annotation.
source/isaaclab/changelog.d/rschmitt_default_cameracfg_renderer.rst	Both :meth: cross-references use get_default_renderer_cfg (wrong name) instead of the actual get_default_render_cfg; will produce broken Sphinx links.

Sequence Diagram

sequenceDiagram
    participant User
    participant CameraCfg
    participant backend_utils
    participant isaaclab_physx

    User->>CameraCfg: CameraCfg(...) [renderer_cfg defaults to None]
    CameraCfg->>CameraCfg: __post_init__()
    alt renderer_cfg is None
        CameraCfg->>backend_utils: get_default_render_cfg()
        backend_utils->>isaaclab_physx: importlib.import_module("isaaclab_physx.renderers")
        isaaclab_physx-->>backend_utils: renderers module
        backend_utils->>isaaclab_physx: IsaacRtxRendererCfg()
        isaaclab_physx-->>backend_utils: IsaacRtxRendererCfg instance
        backend_utils-->>CameraCfg: RendererCfg instance
        CameraCfg->>CameraCfg: self.renderer_cfg = result
    else renderer_cfg already set
        CameraCfg->>CameraCfg: skip lazy import
    end
    CameraCfg-->>User: fully initialized CameraCfg

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