Synchronize distributed FlashInfer autotuning and persist cache

[jiahanc]

Need flashinfer version update

Purpose

Distributed FlashInfer autotuning currently disables the persistent cache and reruns tuning on every server startup. Ranks also profile tactics independently, so GPU timing noise can cause them to select different tactics.

flashinfer-ai/flashinfer#3187 adds set_autotune_process_group, which averages tactic timings across ranks so all ranks select the same tactic.

This PR wires that API into vLLM:

• Broadcast and load an existing cache before distributed autotuning.
• Run autotuning on every rank with synchronized timings over the world CPU process group.
• Save the resulting cache on rank 0.
• Broadcast and load the final cache on every rank.
• Clear the autotune process group in a finally block.

Test Plan

Test Result

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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