[Feat][Plugin] Enable spec decoding for vLLM Plugin

[whx-sjtu]

Motivation

This PR enables spec decode feature for running GLM5 with vLLM+atom.

Technical Details

1. atom_config related bugfix.
2. Fix wrong full_cls_name of different MLA sparse attention backends.
3. Register model architecture and model class for GLM5 MTP
4. Add index_buffer for DeepseekMTP.
5. Adapt full graph of main model with mtp enabled.

Test Plan

Comming soon.

Test Result

1. zai-org/GLM-5.1-FP8

Accuracy test commands:

lm_eval --model local-completions \
        --model_args model=/home/models/GLM-5.1-FP8,base_url=http://localhost:8000/v1/completions,num_concurrent=64,max_retries=3 \
        --tasks gsm8k \
        --num_fewshot 20

Accuracy test result with mtp=3:

|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value |   |Stderr|
|-----|------:|----------------|-----:|-----------|---|-----:|---|-----:|
|gsm8k|      3|flexible-extract|    20|exact_match|↑  |0.9454|±  |0.0063|
|     |       |strict-match    |    20|exact_match|↑  |0.9462|±  |0.0062|

2. deepseek-ai/DeepSeek-R1-0528

Accuracy test commands:

lm_eval --model local-completions \
        --model_args model=/home/models/DeepSeek-R1-0528,base_url=http://localhost:8000/v1/completions,num_concurrent=16,max_retries=3,tokenized_requests=False \
        --tasks gsm8k \
        --num_fewshot 3

Accuracy test result with mtp=3:

|Tasks|Version|     Filter     |n-shot|  Metric   |   |Value |   |Stderr|
|-----|------:|----------------|-----:|-----------|---|-----:|---|-----:|
|gsm8k|      3|flexible-extract|     3|exact_match|↑  |0.9492|±  |0.0060|
|     |       |strict-match    |     3|exact_match|↑  |0.9469|±  |0.0062|

Submission Checklist

• This PR is based on PR [Feat][Plugin] Enable Sparse MLA and GLM-5 for vLLM-ATOM #399 and should be merged after that.

[wuhuikx]

Could you please help attach the accuracy test results on gms8k? Do we support MTP=1 or MTP=1/2/3? How about the acceptance ratio?

[wuhuikx]

I will turn this PR to draft and go through CI after the code review is done.

[whx-sjtu]

Could you please help attach the accuracy test results on gms8k? Do we support MTP=1 or MTP=1/2/3? How about the acceptance ratio?

Sure I will attach the acc results later. Now we support MTP=1/2/3, but the acceptance rate is low (about 20% for first draft token and 0 for other tokens) and I'm working on it.