[Attention] Clean up iRoPE in V1

[LucasWilkinson]

Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

Purpose

With 89cab4d we can actually entirely remove the concept of iRoPE from V1 backends.

Test Plan

lm eval checks; maybe someone with access to a TPU could help me test that? otherwise the change seems simple enough

Test Result

VLLM_ATTENTION_BACKEND=FLASH_ATTN python -m lm_eval --model vllm --model_args pretrained=/home/lwilkinson/local_models/meta-llama--Llama-4-Scout-17B-16E-Instruct,tensor_parallel_size=2,gpu_memory_utilization=0.8,trust_remote_code=True,max_model_len=16384 --tasks ruler --limit 100 --batch_size auto --output_path ./test_fixed_flashinfer.json
...
|Groups|Version|Filter|n-shot|Metric|   |Value |   |Stderr|
|------|------:|------|------|-----:|---|-----:|---|------|
|ruler |      1|none  |      |  4096|↑  |0.9527|±  |   N/A|

(Optional) Documentation Update

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[gemini-code-assist]

Code Review

This pull request effectively refactors the handling of iRoPE by centralizing the logic and removing it from individual V1 attention backends. The changes are clean and align with the stated goal. I've identified one potential issue regarding a removed warning for the CPU backend that could lead to silent correctness issues if iRoPE is not actually supported. Please see the detailed comment.

[mgoin]

LGTM! I'll share it with @yaochengji to make sure TPU is unaffected

[yaochengji]

LGTM, thanks! This change looks good on TPU.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @LucasWilkinson.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @LucasWilkinson.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork