Enable fast path for negative indices

[manuelcandales]

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/15622

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❌ 1 New Failure, 3 Pending, 5 Unrelated Failures

As of commit 7d7d1d6 with merge base 6b545a2 ():

NEW FAILURE - The following job has failed:

• pull / android / run-emulator (gh)
  The process '/usr/bin/sh' failed with exit code 255

FLAKY - The following jobs failed but were likely due to flakiness present on trunk:

• pull / test-binary-size-linux-gcc / linux-job (gh) (similar failure)
  ##[error]The operation was canceled.
• pull / test-setup-linux-gcc / linux-job (gh) (similar failure)
  ##[error]The operation was canceled.

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

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

• pull / test-openvino-linux / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq

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[meta-codesync]

@manuelcandales has exported this pull request. If you are a Meta employee, you can view the originating Diff in D86351194.

[github-actions]

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[manuelcandales]

@pytorchbot cherry-pick --onto release/1.0 -c critical

[pytorchbot]

Cherry picking #15622

The cherry pick PR is at #15657 and it is recommended to link a critical cherry pick PR with an issue. The following tracker issues are updated:

• [v1.0.1] Release Tracker #15534 (comment)

Details for Dev Infra team

Raised by workflow job