[Lane 6 · review] SDPA backward — review SDPA backward #1388

Sub-issue of #1384 ([Feature]: scaledDotProductAttention backward — gradients currently stop at attention).

Lane: 6 · Review
Skill needed: DARC review. Must not be the implementer of Lane 2.
Size: s (a few hours)
Blocked by: #1386, #1387

What to do

1. Wait until the other lanes of this feature are merged or in review.
2. Check the backward rule against the parent issue's Document/Assess sections and the PyTorch reference.
3. Run the tests named in each lane's acceptance list yourself.
4. Confirm the coverage matrix (docs/modules/ROOT/pages/reference/ops-status-matrix.adoc) shows ✅ where the lanes claim it and no @NotImplemented was left behind.

Acceptance

• Review notes posted on the parent issue

Notes

If something here turns out to be wrong or much bigger than the size label, comment on this issue or open a specific bug instead of blocking the parent.

Coverage matrix: docs/modules/ROOT/pages/reference/ops-status-matrix.adoc · How lanes work: Issue taxonomy