Add pytorch-tokenizers as a dependency of ExecuTorch

[larryliu0820]

So that we can pip install executorch and run python code like examples/models/llama/runner/generation.py without installing pytorch-tokenizers separately.

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15492

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⏳ No Failures, 14 Pending

As of commit 6628888 with merge base 5b1004c ():
💚 Looks good so far! There are no failures yet. 💚

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[github-actions]

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[lucylq]

Nice!

[larryliu0820]

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

[pytorchbot]

Cherry picking #15492

Command git -C /home/runner/work/executorch/executorch cherry-pick -x -X theirs 67a94af6adf995d537d4693106d4b91578248741 returned non-zero exit code 1

hint: Recursive merging with submodules currently only supports trivial cases.
hint: Please manually handle the merging of each conflicted submodule.
hint: This can be accomplished with the following steps:
hint:  - go to submodule (extension/llm/tokenizers), and either merge commit 3aada3f
hint:    or update to an existing commit which has merged those changes
hint:  - come back to superproject and run:
hint:
hint:       git add extension/llm/tokenizers
hint:
hint:    to record the above merge or update
hint:  - resolve any other conflicts in the superproject
hint:  - commit the resulting index in the superproject
hint:
hint: Disable this message with "git config set advice.submoduleMergeConflict false"
Failed to merge submodule extension/llm/tokenizers (commits don't follow merge-base)
CONFLICT (submodule): Merge conflict in extension/llm/tokenizers
Auto-merging pyproject.toml
error: could not apply 67a94af6ad... Add pytorch-tokenizers as a dependency of ExecuTorch (#15492)
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config set advice.mergeConflict false"

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