Qualcomm AI Engine Direct - Fix sliding window attention

[DannyYuyang-quic]

Summary

When n_past + n_update exceeds the available_cache_len
it should mask only the remain part.

Test plan

gemma3 CI in TestExampleLLMScript.test_static_llm_model

[pytorch-bot]

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⚠️ 15 Awaiting Approval

As of commit 7238474 with merge base 1b26db1 ():

AWAITING APPROVAL - The following workflows need approval before CI can run:

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• Test ARM Backend (gh)
• Test CoreML Backend (gh)
• Test QNN Backend (gh)
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• Test XNNPACK Backend (gh)

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