Fix unsigned comparison to remove compiler warning on embedded builds

[JakeStevens]

Summary

Internal builds for embedded targets treat type limit warnings as errors.

There is an unsigned comparison against zero, which generates an error.

This simple PR removes it.

Test plan

CI.

Tested in successful internal build

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/12446

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❌ 1 New Failure, 3 Unrelated Failures

As of commit c725729 with merge base dd4488d ():

NEW FAILURE - The following job has failed:

• pull / test-eval_llama-mmlu-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 0adbd94685f39287c59901d3d662425da25d64a0342d651e21a27d6f6c62ec53 /exec failed with exit code 1

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

• pull / test-llama-runner-linux (fp32, xnnpack+quantize_kv, linux.2xlarge, executorch-ubuntu-22.04-clang12) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-models-linux (ic3, xnnpack-quantization-delegation, linux.2xlarge) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)
• pull / test-models-linux (mv2, portable, linux.2xlarge) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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