[Bugfix] Fix potential EAGLE spec decode segfault during graph capture

[mawong-amd]

Purpose

This PR restores the original logic in SpecDecodeBaseProposer's dummy_run function, which was changed after the refactor in #30143. Credits also to @micah-wil for the investigation and fix.

After the refactor, the use_cudagraphs parameter is left unused which incorrectly leads to CUDA graph dispatch where they previously would not have occurred. We are seeing this manifest on MI300X as a segfault during CUDA graph capture with EAGLE spec decode on TP > 1, e.g. by running
vllm serve meta-llama/Llama-3.1-8B-Instruct --max-model-len 2048 -tp 2 --max-num-batched-tokens 2048 --speculative-config='{"method": "eagle", "model": "yuhuili/EAGLE-LLaMA3.1-Instruct-8B", "num_speculative_tokens": 3, "max_model_len": 2048}'
or
vllm serve meta-llama/Llama-4-Scout-17B-16E-Instruct --max-model-len 2048 -tp 2 --max-num-batched-tokens 2048 --speculative-config='{"method": "eagle", "model": "morgendave/EAGLE-Llama-4-Scout-17B-16E-Instruct", "num_speculative_tokens": 3, "max_model_len": 2048}' --load-format dummy --max-num-seqs 1 --compilation-config='{"cudagraph_capture_sizes": [1]}'

This is also leading to failures inside tests like tests/v1/e2e/test_spec_decode.py::test_eagle_correctness.

Test Plan

pytest -sv tests/v1/e2e/test_spec_decode.py -k test_eagle_correctness

Test Result

The test above should pass.

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[gemini-code-assist]

Code Review

This pull request addresses a bug in SpecDecodeBaseProposer.dummy_run where the use_cudagraphs parameter was being ignored, leading to unconditional CUDA graph dispatch and potential segmentation faults. The change correctly restores the intended logic by wrapping the cudagraph_dispatcher.dispatch call in a conditional block that respects the use_cudagraphs flag. When use_cudagraphs is false, it now correctly sets cudagraph_runtime_mode to CUDAGraphMode.NONE. The fix is correct, well-contained, and directly addresses the described bug. The code is clear and I have no further suggestions.

[LucasWilkinson]

LGTM thanks for the fix!