fix: cuda graph issue while running longcat_flash

[tianhaoz95]

Motivation

Currently when we run longcat_flash it will error out with:

[2025-11-25 13:35:50 TP3 EP3] Scheduler hit an exception: Traceback (most recent call last):
  File "/home/zhoutianhao03/projects/sglang/python/sglang/srt/managers/scheduler.py", line 2712, in run_scheduler_process
  ...
  File "/home/zhoutianhao03/projects/sglang/python/sglang/srt/layers/communicator.py", line 187, in fetch_qkv_latent
    assert self.attn_inputs_ is not None
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AssertionError

this is because in #10568 set_attn_inputs() is skipped when qkv_latent_func == None, and later fetch_qkv_latent() will assert on this and fail.

Modifications

Make sure qkv_latent_func is set.

Accuracy Tests

After the change the following launch command runs and output normal tokens:

python3 -m sglang.launch_server \
    --trust-remote-code \
    --model $MODEL_PATH \
    --tp 8 \
    --ep-size 8 \
    --skip-server-warmup \
    --cuda-graph-bs 1 2 3 4 5 6 7 8 \
    --host 0.0.0.0 \
    --port 8080

gsm8k:

Accuracy: 0.950
Invalid: 0.000
Latency: 31.601 s
Output throughput: 386.859 token/s

Benchmarking and Profiling

gsm8k:

Accuracy: 0.950
Invalid: 0.000
Latency: 31.601 s
Output throughput: 386.859 token/s

same as when i revert to before it was broken:

Accuracy: 0.950
Invalid: 0.000
Latency: 34.700 s
Output throughput: 354.785 token/s

Checklist

• Format your code according to the Format code with pre-commit.
• Add unit tests according to the Run and add unit tests.
• Update documentation according to Write documentations.
• Provide accuracy and speed benchmark results according to Test the accuracy and Benchmark the speed.
• Follow the SGLang code style guidance.
• Work with maintainers to merge your PR. See the PR Merge Process

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[tianhaoz95]

/gemini help

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[tianhaoz95]

Reopen to trigger gemini

[tianhaoz95]

gemini triage won't trigger close this one