Add bilinear_pos_embed triton kernel and cache

[zhandaz]

Summary

• Add a fused Triton kernel for bilinear position-embedding interpolation that replaces ~25 small eager-mode CUDA kernels with a single launch
• Add position embedding cache with pre-warming for the top 100 most common grid configurations (~71% MLPerf VLM dataset coverage, ~0.9 GB at BF16).
• Cache is bounded to the pre-defined warmup set to prevent unbounded memory growth; cache misses are computed on-the-fly without caching
• Pre-warming runs automatically after model weight loading

Caching mechanism is borrowed from the PR #33 of @b-mu.

How to enable

The Triton kernel is off by default. To enable:

export VLLM_USE_TRITON_POS_EMBED=1

Pos emb cache size can be controlled by

# Default to 100
export VLLM_POS_EMBED_CACHE_SIZE=0

Files changed

• vllm/envs.py -- register VLLM_USE_TRITON_POS_EMBED
• vllm/model_executor/models/qwen3_vl.py -- Triton kernel, cache infrastructure, warmup grid list

Please review after the next PR on FP8 attn is up. Will run validations of these two PRs together.