[CI unblock] Skip viewergl-fully-black test + fix warp intersphinx 404 (#5538)

## Summary

Two unrelated CI breakages on develop, bundled here so develop turns
green in one PR.

### 1. Skip the failing viewergl test

`test_cartpole_newton_visualizer_viewergl_rgb_motion[physx,newton]`
started returning all-black frames on develop after
`nvcr.io/nvidian/isaac-sim:latest-develop` flipped to a Kit 110.1.1 +
USD 25.11 base. The failure has been deterministic across multiple PRs
(#5523, #5495, #5408, …).

Investigation so far has ruled out:
- PR #5521 (revert in
#5539 still failed)
- Newton 1.0 → 1.2.0rc2 viewer code regression (only 7-line addition;
ViewerGL alone yields 1.08M nonzero pixels)
- warp 1.12 → 1.13 RegisteredGLBuffer ABI (byte-identical)
- Module-load side effects of `isaaclab_physx.renderers`
- CUDA-GL interop (PR #5540 diagnostic confirms direct CPU FBO readback
also returns zeros, with `GL_NO_ERROR`)
- GL context-currency (PR #5541 H6 attempt: still fails)
- GL/CUDA sync (PR #5542 H4 attempt: still fails)

Diagnostic output (PR #5540 v2):
```
[VIZDIAG] fbo=c_uint(8)  pbo=None  size=600x600
[VIZDIAG] glGetError before: GL_NO_ERROR
[VIZDIAG] CPU-readback: nonzero=0/1080000  max=0  err=GL_NO_ERROR
[VIZDIAG] PBO-result: nonzero=0/1080000  max=0
```

The FBO itself is empty — Newton's pyglet/EGL renderer is not depositing
pixels under Kit 110.1.1, even though `tiled_camera_rgb_non_black` (Kit
RTX path) on the same env passes. Underlying root cause still being
chased; this PR ships the skip to unblock develop.

### 2. Fix warp intersphinx 404 in docs build

`https://nvidia.github.io/warp/objects.inv` started returning 404 —
Warp's `objects.inv` only lives at `/stable/` and `/latest/` now. With
Sphinx's `warnings_treated_as_errors`, the broken intersphinx fetch
fails the docs build on every PR. Pinning to `/stable/` (matches the
existing PyTorch `/docs/2.11/` workaround pattern in the same file).

Verified `https://nvidia.github.io/warp/stable/objects.inv` returns 200.

## Test plan

- [x] CI `isaaclab_visualizers` on this branch — was passing earlier
with the skip; will re-verify with the bundled docs fix
- [ ] CI `Build Latest Docs` on this branch — must turn green (was
failing on every recent PR before this fix)

## Re-enable plan

Once the underlying viewergl bug is identified and fixed, drop the
`@pytest.mark.skip` decorator and remove the
`jichuanh-disable-viewergl-flaky.skip` fragment.

Author: hujc7

docs/conf.py

@@ -140,7 +140,8 @@
     "torch": ("https://docs.pytorch.org/docs/2.11/", None),
     "isaacsim": ("https://docs.isaacsim.omniverse.nvidia.com/6.0.0/py/", None),
     "gymnasium": ("https://gymnasium.farama.org/", None),
-    "warp": ("https://nvidia.github.io/warp/", None),
+    # NOTE: pinned to /stable/ because /objects.inv at the root currently 404s
+    "warp": ("https://nvidia.github.io/warp/stable/", None),
     "omniverse": ("https://docs.omniverse.nvidia.com/dev-guide/latest", None),
 }
 

source/isaaclab/changelog.d/jichuanh-fix-warp-intersphinx.rst

@@ -0,0 +1,5 @@
+Fixed
+^^^^^
+
+* Fixed Sphinx docs build failing due to ``https://nvidia.github.io/warp/objects.inv`` returning 404.
+  Pinned the ``warp`` intersphinx mapping to ``/stable/``, which is where the inventory now lives.

source/isaaclab_visualizers/changelog.d/jichuanh-disable-viewergl-flaky.skip

@@ -522,6 +522,22 @@ def test_cartpole_newton_visualizer_tiled_camera_rgb_non_black(
 
 
 @pytest.mark.isaacsim_ci
+@pytest.mark.skip(
+    reason=(
+        "ViewerGL.get_frame returns a fully-black 600x600x3 buffer in CI on the current "
+        "Isaac Sim image + Newton 1.2.0rc2 + warp-lang 1.13 cohort. Failure is "
+        "deterministic across two consecutive reruns of the same SHA and reproduces on "
+        "every PR that touches the rendering / camera / sensor / USD stack (5 PRs hit it "
+        "in the last 100 build.yaml runs); zero failures on PRs outside that scope. "
+        "Investigation ruled out: rc1->rc2 viewer code diff (7-line image_logger.clear "
+        "only), wp.RegisteredGLBuffer API (byte-identical 1.12 vs 1.13), pure flakiness "
+        "(deterministic), and the bump cohort alone (warp-1.12 branches both pass and "
+        "fail). Strongest remaining hypothesis: a CUDA-OpenGL interop init-order "
+        "fragility in the PBO + glReadPixels + RegisteredGLBuffer.map path that gets "
+        "tipped by any source change perturbing GL/CUDA bring-up. Re-enable once root "
+        "cause is identified."
+    )
+)
 @pytest.mark.parametrize("backend_kind", ["physx", "newton"])
 def test_cartpole_newton_visualizer_viewergl_rgb_motion(backend_kind: str, caplog: pytest.LogCaptureFixture) -> None:
     """Newton GL (``ViewerGL.get_frame``): full motion steps, last frame non-black; early vs late differ; logs."""