[V0 deprecation] Remove V0 CPU/XPU/TPU backends

[WoosukKwon]

This PR is part of V0 deprecation; To begin with, we will delete the hardware backends in vLLM V0 that has already been migrated to vLLM V1 (i.e., CPU, XPU, and TPU).

[github-actions]

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[gemini-code-assist]

Summary of Changes

Hello @WoosukKwon, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request initiates the deprecation of V0 hardware backends by removing the V0 implementation for the TPU backend. The changes streamline the codebase by eliminating legacy V0 paths, ensuring that the TPU backend exclusively utilizes the V1 architecture for attention, worker management, and other functionalities. This simplifies maintenance and aligns the TPU support with the newer V1 design.

Highlights

• TPU Backend V0 Deprecation: The pull request removes the V0 implementation for the TPU backend, enforcing the exclusive use of the V1 architecture for all TPU operations.
• Enforced V1 Pallas Attention Backend: The logic for selecting the attention backend on TPU has been simplified to unconditionally use vllm.v1.attention.backends.pallas.PallasAttentionBackend, eliminating the legacy V0 path.
• V1 Worker Class Enforcement: The TPU worker class selection now defaults to vllm.v1.worker.tpu_worker.TPUWorker, removing the V0 worker and multi-step scheduling options previously available for TPU.
• Removal of V0-Specific Checks: V0-specific conditional logic and imports related to asynchronous output support and guided decoding validation have been removed, streamlining the codebase and reducing complexity.

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[gemini-code-assist]

Code Review

This pull request aims to deprecate V0 backends, and the changes in vllm/platforms/tpu.py effectively remove the V0-specific logic for the TPU platform. The code has been simplified by removing conditional paths based on VLLM_USE_V1, making the V1 implementation the default and only path. The refactoring is clean, improves maintainability, and correctly reflects the goal of supporting only V1 for TPUs. I found no issues with the changes.

[WoosukKwon]

cc @jikunshang @yaochengji @bigPYJ1151

Could you please take a look?

[simon-mo]

There might be tests still requires V0, which you can temporarily disable.

For example

vllm/.buildkite/scripts/hardware_ci/run-cpu-test.sh

Lines 69 to 72 in 0ec3779

	docker exec cpu-test-"$NUMA_NODE" bash -c "
	set -e
	VLLM_USE_V1=0 pytest -s -v \
	tests/quantization/test_ipex_quant.py"

cc @bigPYJ1151

[bigPYJ1151]

@WoosukKwon @simon-mo I will adapt the CPU part based on this PR to make the tests pass. BTW, will this PR be included in the incoming release?

[jikunshang]

LGTM for xpu part. thanks!

[WoosukKwon]

will this PR be included in the incoming release?

@bigPYJ1151 Good question. Let me ask around.

[mergify]

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

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