Qualcomm AI Engine Direct - fix attention sink feature

[DannyYuyang-quic]

Summary

Fix Attention sink feature

Test plan

TestExampleLLMScript.test_attention_sink in ./backends/qualcomm/tests/test_qnn_delegate.py

[pytorch-bot]

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[DannyYuyang-quic]

Hi @abhinaykukkadapu,
After we removed prefill calibration in LLM, we accidentally removed the indexing for the attention sink feature. This PR adds it back.
Please have a look.
Thanks!

[DannyYuyang-quic]

@pytorchbot label "release notes: qualcomm"