Add check for negative index in cpu kernel op_index

[pytorchbot]

Fast path cannot be taken when the index is negative in op_index. It results in a failure in check_fast_path_args. Add a check to check_fast_path_conditions for negative index and return false if one is encountered.

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15646

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❌ 3 New Failures, 2 Cancelled Jobs

As of commit 157da77 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 d53de65668482d84d3de1de4d2ee678927f56439cc6ce9f5e1e8fff823bcccd8 /exec failed with exit code 1
• pull / test-qnn-wheel-packages-linux (3.11) / linux-job (gh)
  RuntimeError: Command docker exec -t b04c9b587d4dea8d06def87383f153edc7e4299ac59f555feedadd9384ab46b1 /exec failed with exit code 1
• pull / test-qnn-wheel-packages-linux (3.12) / linux-job (gh)
  RuntimeError: Command docker exec -t c0d6e7d50a18b106b217b7a1c81e29c63f61922c62be365925029bd58a2d177c /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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[GregoryComer]

Something in the QNN jobs is pinning a nightly build of torch that is out of retention. I'll take a look.