Bump tokenizers to 4ed91cc

[GregoryComer]

Summary

Update the tokenizers dependency to 4ed91cc to pick up the Windows changes. Note that this also involves updating the submodule URL, as it's been recently migrated from pytorch-labs to meta-pytorch.

Test plan

CI

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/13888

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❌ 4 New Failures, 1 Unrelated Failure

As of commit d70ace8 with merge base 0b0e2dc ():

NEW FAILURES - The following jobs have failed:

• Build documentation / build (buck2) / Build doc (gh)
  At least one of the pre-conditions you specified did not hold
• pull / android / run-emulator (gh)
  The process '/usr/bin/sh' failed with exit code 255
• trunk / test-huggingface-transformers-coreml (qwen3-1.7b|xnnpack|--quantize) / macos-job (gh)
  RuntimeError: Command bash /Users/runner/work/_temp/exec_script failed with exit code 139
• trunk / test-models-macos-coreml (resnet50) / macos-job (gh)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 139

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

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

• pull / unittest-nxp-neutron / linux-job (gh) (trunk failure)
  test_split_group_convolution__applied_by_default

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