Add EarlyTom video token compression for Qwen2.5-VL.

[zhangj1an]

Port of EarlyTom (arXiv:2605.30010, github.com/viridisGreen/EarlyTom)
from LLaVA-OneVision/SigLIP to vLLM's Qwen2.5-VL, riding the EVS
pruning infrastructure (--video-pruning-rate) so the processor-side
placeholder count contract is preserved exactly.

• In-encoder temporal frame merging (EMA cosine segmentation + frame mixing) at configurable ViT layers, with full rebuild of cu_seqlens, window cu_seqlens, rope tables and merger reverse indices
• Attention saliency recomputed from QK^T at the last full-attention layer (flash-attention safe, query-chunked, TP all-reduced)
• Outer compression: global attention top-k dominant tokens + DPC-KNN contextual merging for segment-boundary frames; local-window top-k for static frames; exact-budget allocator + retention mask drives EVS mrope recomputation
• Inner (FastV-style) LLM compression intentionally not ported: paged KV cache assumes fixed prompt length
• Enabled via VLLM_EARLYTOM=1 (+ VLLM_EARLYTOM_* hyperparams)

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Purpose

Test Plan

Test Result

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