Avoid CPU-GPU synchronization due to PR: #4514. cc: @zhyncs @merrymercy

ref #4577

Motivation

Fix FlashMLA cudagraph config.

And some result for using FlashMLA Decode

python3 -m sglang.launch_server --model-path deepseek-ai/DeepSeek-V3 --trust-remote --tp 8 --attention-backend flashinfer --page-size 64 --enable-flashmla

python3 -m sglang.bench_one_batch_server --model None --base-url http://localhost:30000 --batch-size 1 2 4 8 16 32 --input-len 256 --output-len 256

• FlashMLA

batch size: 1
latency: 8.38 s
output throughput: 30.56 token/s
(input + output) throughput: 61.13 token/s
batch size: 2
latency: 10.36 s
output throughput: 49.43 token/s
(input + output) throughput: 98.86 token/s
batch size: 4
latency: 10.17 s
output throughput: 100.70 token/s
(input + output) throughput: 201.39 token/s
batch size: 8
latency: 11.70 s
output throughput: 175.04 token/s
(input + output) throughput: 350.08 token/s
batch size: 16
latency: 12.70 s
output throughput: 322.57 token/s
(input + output) throughput: 645.14 token/s
batch size: 32
latency: 15.25 s
output throughput: 537.16 token/s
(input + output) throughput: 1074.32 token/s

• FlashInfer

batch size: 1
latency: 8.82 s
output throughput: 29.02 token/s
(input + output) throughput: 58.04 token/s
batch size: 2
latency: 10.17 s
output throughput: 50.37 token/s
(input + output) throughput: 100.74 token/s
batch size: 4
latency: 9.94 s
output throughput: 103.06 token/s
(input + output) throughput: 206.12 token/s
batch size: 8
latency: 12.07 s
output throughput: 169.72 token/s
(input + output) throughput: 339.44 token/s
batch size: 16
latency: 12.87 s
output throughput: 318.35 token/s
(input + output) throughput: 636.70 token/s
batch size: 32
latency: 15.26 s
output throughput: 536.66 token/s
(input + output) throughput: 1073.32 token/s

With longer sequences, the performance advantages of FlashMLA become more significant.

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.
• For reviewers: If you haven't made any contributions to this PR and are only assisting with merging the main branch, please remove yourself as a co-author when merging the PR.
• Please feel free to join our Slack channel at https://slack.sglang.ai to discuss your PR.

hi @yinfan98 I want to know what gpu the above test results are based on. Is there test results of compared with triton-backend