Share mutable buffers in export llama

[lucylq]

Summary

Test plan

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/17639

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❌ 5 New Failures, 1 Cancelled Job, 1 Unrelated Failure

As of commit f9aa8f9 with merge base 23f4fb4 ():

NEW FAILURES - The following jobs have failed:

• pull / test-mediatek-models-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 8a8c7e25cb6166672ed67aa2d1142de76e5f5e483f33cb38618d6f3c81e75646 /exec failed with exit code 2
• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t efccb7afe4c704b0b3f27f95d0494fe24a0b73c49947492d2ade4c61c71bd1f3 /exec failed with exit code 139
• pull / test-openvino-linux / linux-job (gh)
  RuntimeError: Command docker exec -t 1950e9ec742a5d5feafb4481aa68b2657c329e3c75b7f510198c91cfaed4c2a5 /exec failed with exit code 1
• pull / unittest-arm-backend-with-no-deps (test_pytest_ops_tosa) / linux-job (gh)
  RuntimeError: Command docker exec -t 92039eab21f44c4987cab65bb0b81b79d39bddd3baf100b047b6015dfd677822 /exec failed with exit code 1
• pull / unittest-buck / linux / linux-job (gh)
  RuntimeError: Command docker exec -t e7135d0d50d17c58225f6799ac84ee77bb74b443078731c0e8be161c69af32ae /exec failed with exit code 3

CANCELLED JOB - The following job was cancelled. Please retry:

• Check Labels / Check labels (gh)

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-buck / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 3

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