Fix OOB write to S_data in batched linalg_svd (#18625)

[abeakkas]

Summary:

S has 3 elements per batch (diagonal of 3x3), but was indexed with the same offset as U and Vh (9 elements per batch), causing out-of-bounds writes and corrupted singular values for batch_size > 1.

Reviewed By: DrJessop

Differential Revision: D99002350

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/18625

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❌ 2 New Failures, 2 Unrelated Failures

As of commit 560aba9 with merge base fcccda3 ():

NEW FAILURES - The following jobs have failed:

• Cadence Build & Test / cpu-test / test-ops / test-ops (gh)
  examples/cadence/operators/test_g3_ops.py::ATenOpTestCases::test_g3_neg_out_5
• pull / unittest-editable / macos / macos-job (gh)
  export/tests/test_target_recipes.py::TestTargetRecipes::test_mv3_model

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / unittest / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  ##[error]The operation was canceled.

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[meta-codesync]

@abeakkas has exported this pull request. If you are a Meta employee, you can view the originating Diff in D99002350.

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