[WIP] Support double sparsity

[andy-yang-1]

Motivation

• Support double sparsity (post-training sparse attention) for long context inference in SGLang
• See paper

Modifications

• Add triton implementation in sglang/python/sglang/srt/layers/sparse_decode_attention.py
• Add serving-related parts

Checklist

• [ ] Format your code according to the Contributor Guide.
• [ ] Add unit tests as outlined in the Contributor Guide.
• [ ] Update documentation as needed, including docstrings or example tutorials.