[Breakable Graph] add eager break for unified attention

[zhenwei-intel]

Purpose

Add eager break support for unified attention in breakable CUDA graph, follow up #42304

Test Plan

Online benchmark on 4090D

Config: 1024 input tokens, 256 output tokens, 16 concurrency

--- Performance Results ---
Model                          Mode                      req/s  out_tok/s    ttft_ms    tpot_ms
--------------------------------------------------------------------------------------------
llama3-8b                      eager                      1.20     613.32     661.02      22.64
llama3-8b                      compile-only               1.21     620.80     664.49      22.35
llama3-8b                      full_and_piecewise         1.23     628.18     660.44      22.09
llama3-8b                      breakable-cudagraph        1.21     621.24     662.75      22.34
llama3-8b-tp4                  eager                      2.31    1184.53     517.26      11.33
llama3-8b-tp4                  compile-only               2.41    1235.01     521.39      10.83
llama3-8b-tp4                  full_and_piecewise         2.74    1401.45     548.74       9.56
llama3-8b-tp4                  breakable-cudagraph        2.72    1394.69     541.41       9.62
qwen3-30b-a3b                  eager                      0.78     399.77     410.93      35.33
qwen3-30b-a3b                  compile-only               0.75     384.61     421.48      36.71
qwen3-30b-a3b                  full_and_piecewise         1.79     914.67     484.59      15.33
qwen3-30b-a3b                  breakable-cudagraph        1.84     944.29     511.77      14.93
deepseek-v2-lite               eager                      0.79     406.86     300.64      34.73
deepseek-v2-lite               compile-only               0.74     379.10     328.31      37.32
deepseek-v2-lite               full_and_piecewise         1.68     859.28     365.88      16.87
deepseek-v2-lite               breakable-cudagraph        1.69     864.51     297.19      16.90

Breakable CUDA graph achieves parity with full-and-piecewise cudagraph

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

[zhenwei-intel]

cc @ZJY0516 @WoosukKwon