Avoid unused quantized SDPA scratch allocation

[JacobSzwejbka]

Summary

• allocate QDQ scratch space only for quantized SDPA
• leave floating-point SDPA without the unused per-thread kv_split_size * head_size buffer
• preserve the existing quantized execution path

Stack

• stacked on Optimize CPU SDPA for ring attention #22358
• this PR contains only the quantized scratch-memory optimization

Testing

• existing CPU SDPA correctness coverage in the parent stack
• verified the stacked diff is limited to op_sdpa_impl.h

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/22417

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❌ 3 New Failures, 6 Unrelated Failures

As of commit 64cb0bc with merge base 6755ea3 ():

NEW FAILURES - The following jobs have failed:

• pull / test-multimodal-linux (gemma3-4b) / linux-job (gh)
  RuntimeError: Command docker exec -t 20c5adf50d0b2b1dd27088398b380153763627f576b9d91c7c869fe7cd8c6b70 /exec failed with exit code 1
• pull / unittest / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 809bce19e5e76b4284016523dad3b6654c1697de601d976191a4f5758b9d7a8d /exec failed with exit code 1
• pull / unittest-editable / linux / linux-job (gh)
  RuntimeError: Command docker exec -t 660461a5b303c41ac6ac49442d33c8ad5c18e815fd193b3f9632766c45703999 /exec failed with exit code 1

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

• pull / test-moshi-linux / linux-job (gh) (matched linux rule in flaky-rules.json)
  E: Failed to fetch http://security.ubuntu.com/ubuntu/pool/main/libs/libssh/libssh-gcrypt-4_0.9.6-2ubuntu0.22.04.7_amd64.deb 404 Not Found [IP: 91.189.92.23 80]
• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (matched linux rule in flaky-rules.json)
  The runner has received a shutdown signal. This can happen when the runner service is stopped, or a manually started runner is canceled.
• pull / unittest-editable / macos / macos-job (gh) (similar failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1

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

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

• pull / unittest / macos / macos-job (gh) (trunk failure)
  RuntimeError: Command bash /Users/ec2-user/runner/_work/_temp/exec_script failed with exit code 1
• pull / unittest / windows / windows-job (gh) (trunk failure)
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)

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