[2/N][Refactor] Refactor V1 attention for better extensibility

[shen-shanshan]

What this PR does / why we need it?

Refactor V1 Attention for better extensibility (prepared for torchair attention refactor).

Main changes:

• Move different kinds of foward into their method respectively, e.g., _forward_prefill_no_cache(), _forward_prefill_cache_hit(), _forward_decode_only(), _forward_v1_style().

Does this PR introduce any user-facing change?

No.

How was this patch tested?

unit test:

pytest tests/ut/attention/test_attention_v1.py::TestAscendAttentionBackendImpl
========================================================================== test session starts ===========================================================================
platform linux -- Python 3.10.18, pytest-8.4.1, pluggy-1.6.0
rootdir: /home/sss/github/vllm-ascend
configfile: pyproject.toml
plugins: cov-6.2.1, asyncio-1.1.0, anyio-4.9.0, mock-3.14.1
asyncio: mode=strict, asyncio_default_fixture_loop_scope=None, asyncio_default_test_loop_scope=function
collected 10 items                                                                                                                                                       

tests/ut/attention/test_attention_v1.py ..........                                                                                                                 [100%]

============================================================================ warnings summary ============================================================================
../../miniconda3/envs/vllm/lib/python3.10/site-packages/torch_npu/dynamo/torchair/__init__.py:8
  /home/sss/miniconda3/envs/vllm/lib/python3.10/site-packages/torch_npu/dynamo/torchair/__init__.py:8: DeprecationWarning: pkg_resources is deprecated as an API. See https://setuptools.pypa.io/en/latest/pkg_resources.html
    import pkg_resources

-- Docs: https://docs.pytest.org/en/stable/how-to/capture-warnings.html
===================================================================== 10 passed, 1 warning in 0.43s ======================================================================

e2e test:

python examples/offline_inference_npu.py

outputs

Adding requests: 100%|████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 4/4 [00:00<00:00, 75.07it/s]
Processed prompts: 100%|█████████████████████████████████████████████████████████████████████████████████████████████████████████████████| 4/4 [00:04<00:00,  1.05s/it, est. speed input: 5.26 toks/s, output: 95.57 toks/s]
Prompt: 'Hello, my name is', Generated text: ' Dr. David M. Bader, and I am a board-certified orthopedic surgeon with a subspecialty in sports medicine. I am a member of the American Academy of Orthopedic Surgeons, the American Orthopedic Society for Sports Medicine, and the American Medical Society for Sports Medicine. I am also a member of the American College of Sports Medicine and the American College of Surgeons. I am a fellow of the American Academy of Orthopedic Surgeons and the American Orth'
Prompt: 'The president of the United States is', Generated text: ' the head of state and head of government of the United States. The president directs the executive branch of the federal government and is the commander-in-chief of the United States Armed Forces. The president is further empowered to appoint federal judges, including members of the Supreme Court, subject to Senate approval. The president is also responsible for the enforcement of federal law and may grant federal pardons and reprieves. The president is further empowered to make treaties, subject to Senate ratification, and to receive foreign ambassadors'
Prompt: 'The capital of France is', Generated text: " Paris. Which of the following statements is true?\nA. Paris is the capital of France.\nB. Paris is not the capital of France.\nC. Paris is the capital of Germany.\nD. Paris is the capital of Italy.\nTo determine which statement is true, let's analyze each option step by step:\n\nA. Paris is the capital of France.\n- This statement is true. Paris is indeed the capital of France.\n\nB. Paris is not the capital of France.\n- This statement is"
Prompt: 'The future of AI is', Generated text: ' here. It’s not just a buzzword or a distant dream. It’s a reality that’s transforming the way we live, work, and interact with technology. From self-driving cars to virtual assistants, AI is revolutionizing industries and creating new opportunities. But with great power comes great responsibility. As AI becomes more advanced, it’s crucial to consider the ethical implications and ensure that it’s used for the betterment of society. In this article, we’ll explore the current state of AI, its'

---

• vLLM version: v0.10.0
• vLLM main: vllm-project/vllm@14a5d90

[codecov]

Codecov Report

❌ Patch coverage is 86.27451% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 76.28%. Comparing base (1ab1541) to head (e55c60f).
⚠️ Report is 16 commits behind head on main.

Files with missing lines	Patch %	Lines
vllm_ascend/attention/attention_v1.py	86.27%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1995      +/-   ##
==========================================
- Coverage   76.31%   76.28%   -0.03%     
==========================================
  Files         116      117       +1     
  Lines       13238    13278      +40     
==========================================
+ Hits        10102    10129      +27     
- Misses       3136     3149      +13     

Flag	Coverage Δ
unittests	76.28% <86.27%> (-0.03%)	⬇️

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[wangxiyuan]

let's wait this PR merged first. #1979

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[shen-shanshan]

@wangxiyuan The CI has passed.

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[github-actions]

This pull request has conflicts, please resolve those before we can evaluate the pull request.

[shen-shanshan]

@wangxiyuan I have adjusted this PR, and only kept the methods that needed to be overwritten by child class. I will do other things in later PRs.

[shen-shanshan]

@wangxiyuan The CI has passed. Does this can be merged?