[Feature] Qwen3 VL eagle3 support

[dcw02]

Motivation

** Draft PR. This is currently WIP **

Add eagle3 support for qwen3_vl and qwen3_vl_moe models.

Modifications

Related Issues

Accuracy Test

Benchmark & Profiling

Checklist

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[dcw02]

Training finishes for tp size 1 and both sdpa/flex attention backends. Loss/acc curves look ok, going to add qwen3_vl_moe eagle3 support into sglang/vllm (so I can eval) before adding tp size > 1 support.