Add Eagle-3 Qwen support (follow-up to #20436)

[rahul-tuli]

Summary

This PR adds support for Eagle-3 speculative decoding with Qwen models, as a follow-up to vllm-project#20436.

The changes enable Qwen models to work with Eagle-3 draft models in speculators format.

Related Issues

• Follows up on feat: Add support for speculators Eagle checkpoints vllm-project/vllm#20436 which added initial Eagle speculators support
• This PR is based on the feat/speculators-eagle-support branch

Changes

• Added Qwen support to Eagle-3 implementation
• Proper handling of Qwen model architecture in Eagle-3 speculative decoding

Testing

Tested with the following verification script:

#\!/usr/bin/env python3
"""
Verification script for Qwen Eagle-3 support.
"""

from vllm import LLM, SamplingParams

# Initialize vLLM with Qwen model and Eagle-3 draft model
llm = LLM(
    model="Qwen/Qwen2.5-7B-Instruct",
    speculative_config={
        "method": "eagle",
        "model": "nm-testing/Qwen3-8B-Eagle3-speculators-converted",
        "num_speculative_tokens": 5
    },
    max_model_len=1024,
    enforce_eager=True
)

# Test generation
outputs = llm.generate(
    ["The future of AI is"], 
    SamplingParams(max_tokens=20, temperature=0)
)

print(f"Generated: {outputs[0].outputs[0].text}")
print("✅ Eagle-3 with Qwen works\!")

Output confirms that Eagle-3 speculative decoding works correctly with Qwen models.

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[gemini-code-assist]

Code Review

This PR adds support for Eagle-3 speculative decoding with Qwen models. The return type hint in Qwen2Model.forward is incorrect and should be addressed.