fix draft cuda graph capture failure

[zhyncs]

Motivation

fix Capture cuda graph failed: mat1 and mat2 must have the same dtype, but got Float and Half

python3 -m sglang.launch_server --model meta-llama/Meta-Llama-3-8B-Instruct  \
--speculative-algo EAGLE --speculative-draft lmzheng/sglang-EAGLE-LLaMA3-Instruct-8B  \
--speculative-num-steps 5 --speculative-eagle-topk 8 --speculative-num-draft-tokens 64 \
--disable-radix-cache --mem-fraction 0.7 --cuda-graph-max-bs 32 --dtype bfloat16

https://huggingface.co/meta-llama/Meta-Llama-3-8B-Instruct/blob/main/config.json#L23
https://huggingface.co/lmzheng/sglang-EAGLE-LLaMA3-Instruct-8B/blob/main/config.json#L19

We should explicitly specify --dtype when the target model and draft model use different dtype (such as bfloat16 and float16).

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.

[zhyncs]

fix #3395