Update extend/decode attention kernel for CPU in sgl-kernel and add UTs

[yanbing-j]

Motivation

This PR is a follow-up on #2807 and #5150 to update extend/decode attention kernel for CPU. We fuse set_kv_buffer in decode attention, in order to reduce the overhead. We also add correspondig UTs test_extend.py/test_decode.py for extend/decode attention kernels for CPU.

Modifications

Checklist

• Format your code according to the Code Formatting with Pre-Commit.
• Add unit tests as outlined in the Running Unit Tests.
• Update documentation / docstrings / example tutorials as needed, according to Writing Documentation.
• Provide throughput / latency benchmark results and accuracy evaluation results as needed, according to Benchmark and Profiling and Accuracy Results.
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