[ET-VK][ez] Don't copy zeros for cache tensors

[SS-JIA]

Stack from ghstack (oldest at bottom):

• -> [ET-VK][ez] Don't copy zeros for cache tensors #15580
• [ET-VK][ez] SDPA don't branch based on whether bounds check needed #15579
• [ET-VK][ez] Align SDPA attention weights S dim to the next multiple of 4 #15578
• [ET-VK][ez] Update SDPA test to be able to test different SDPA modes #15577
• [ET-VK][ez] Refactor yaml configs for SDPA shaders #15576

Currently, cache tensors for SDPA are prepacked even though the mutable buffer data just contains zeros.

For fused SDPA, this step can be skipped.

Differential Revision: D86226137

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/15580

• 📄 Preview Python docs built from this PR

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

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

As of commit e99ada1 with merge base bde6b11 ():

NEW FAILURES - The following jobs have failed:

• Test CUDA Builds / export-model-cuda-artifact (google, gemma-3-4b-it, non-quantized) / linux-job (gh)
  RuntimeError: Command docker exec -t 46999ad83be1e2a0f6475ae5890b1b63dead20e60f8abbf54c8cd53d1685a47c /exec failed with exit code 1
• Test CUDA Builds / test-models-cuda (linear) / linux-job (gh)
  RuntimeError: Command docker exec -t 27cf6b1d2b4f08a57e27e34378e937bf8d66cbe9b12d37844c159390e5978f97 /exec failed with exit code 1

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

• Check Labels (gh)

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

• pull / test-setup-linux-gcc / linux-job (gh) (similar failure)
  ##[error]The operation was canceled.

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

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

• pull / test-binary-size-linux-gcc / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / test-openvino-linux / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq

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.