Initial optimization of paged attention

[scxiao]

This PR includes two initial optimizations of the paged attention

• Unrolling the main loop in the triton mma kernel by 2
• some magic changes to simplify computation.

[CLAassistant]

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2 out of 3 committers have signed the CLA.

✅ Guo-Peilin
✅ scxiao
❌ shengnxu
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