Enable use_sdpa_with_kv_cache in the qwen3_5 example config

[john-rocky]

Fixes #22045.

use_sdpa_with_kv_cache: False in this config costs roughly 2x decode. @JakeStevens confirmed in the issue there is no known reason to keep it off, and the lfm2 config — the same hybrid attention/conv architecture — already sets it True.

Measured on Qwen3.5-2B, XNNPACK, 8da4w + 8-bit embedding, the same checkpoint exported twice with only this line different (Mac arm64, token-by-token prefill then greedy decode, same prompt, 32 new tokens):

	decode
use_sdpa_with_kv_cache: False	8.20 tok/s
use_sdpa_with_kv_cache: True	16.64 tok/s

That it is the same model rather than a faster different one: three of four prompts decode token-identically (the fourth diverges inside <think>, a near-tie either way), first-step logits corr 0.995 with the same top-1, and at 338 tokens of context corr 0.990 with the same top-1 and top-2. File size is unchanged (1483 MB both).

cc @mergennachin @iseeyuan @lucylq @helunwencser @tarun292 @kimishpatel @jackzhxng

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/22390

• 📄 Preview Python docs built from this PR

Note: Links to docs will display an error until the docs builds have been completed.

❌ 3 New Failures, 2 Unrelated Failures, 1 Unclassified Failure

As of commit 43a8387 with merge base f5102b8 ():

NEW FAILURES - The following jobs have failed:

• pull / unittest / linux / linux-job (gh)
  pytorch_tokenizers
• pull / unittest-buck / linux / linux-job (gh)
  pytorch_tokenizers
• pull / unittest-editable / linux / linux-job (gh)
  pytorch_tokenizers

UNCLASSIFIED FAILURE - DrCI could not classify the following job because the workflow did not run on the merge base. The failure may be pre-existing on trunk or introduced by this PR:

• periodic (gh) (this job did not run on the merge base, so DrCI cannot tell whether the failure is pre-existing)

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / unittest-editable / windows / windows-job (gh) (detected as infra flaky with no log or failing log classifier)

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

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

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (trunk failure)

This comment was automatically generated by Dr. CI and updates every 15 minutes.

[github-actions]

This PR needs a release notes: label

If your change should be included in the release notes (i.e. would users of this library care about this change?), please use a label starting with release notes:. This helps us keep track and include your important work in the next release notes.

To add a label, you can comment to pytorchbot, for example
@pytorchbot label "release notes: none"

For more information, see
https://github.com/pytorch/pytorch/wiki/PyTorch-AutoLabel-Bot#why-categorize-for-release-notes-and-how-does-it-work.