Enable fast path for negative indices

[pytorchbot]

Fast path was broken for negative indices (see #15285)
Because of this, #15366 disabled the fast path when the index tensor had negative indices.
In this PR we fix the bug, and re-enable the fast path for negative indices.

Fixes #15285

Differential Revision: D86351194

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15657

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❌ 3 New Failures, 2 Cancelled Jobs

As of commit 97debba with merge base e0dda90 ():

NEW FAILURES - The following jobs have failed:

• pull / test-qnn-wheel-packages-linux (3.10) / linux-job (gh)
  RuntimeError: Command docker exec -t f701a6b2f8a3397420b149e411022a823793f83cfcd47b7896e4c52ead508787 /exec failed with exit code 1
• pull / test-qnn-wheel-packages-linux (3.11) / linux-job (gh)
  RuntimeError: Command docker exec -t 338a16625af49bb010e120e97f03c57d594b3181d7ad6506df13e79c6fc59e27 /exec failed with exit code 1
• pull / test-qnn-wheel-packages-linux (3.12) / linux-job (gh)
  RuntimeError: Command docker exec -t b88365fce281a3367d15f086f4072823910befa229c438b421d28cf47676dfc3 /exec failed with exit code 1

CANCELLED JOBS - The following jobs were cancelled. Please retry:

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

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