[Misc][PCP&DCP] relocate PCP feature check

[pisceskkk]

Recently, vllm-ascend has been advancing the main-to-main adaptation work with the vllm repository. We have now fully adapted PCP and other features on vllm-ascend, so we believe it is necessary to add essential feature adaptation flags for the attention backend and modify the logic of the PCP feature check code to ensure that the functional switches for GPU and NPU do not interfere with each other.

Modifications:​

• Consolidate all PCP-related feature check code in the same location as DCP checks. As backend functionalities are progressively completed, the relevant code can be deleted based on the situation.
• Add feature adaptation flags for the attention backend. Subsequent PRs for backend adaptation can enable or disable the corresponding flags based on development progress.

CC @LucasWilkinson @FENP @LookAround0301 @zhenwenqi2024

[gemini-code-assist]

Code Review

This pull request effectively relocates the feature checks for Prefill Context Parallelism (PCP) to a more consolidated and appropriate location, improving code organization and clarity. The introduction of new feature flags in the attention backend's abstract class is a good step towards more granular control over backend capabilities. While the refactoring is well-executed, I've identified one high-severity issue where a compatibility check was incorrectly scoped, which could lead to runtime errors in specific distributed configurations.

[chatgpt-codex-connector]

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[mergify]

Hi @pisceskkk, the pre-commit checks have failed. Please run:

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[LucasWilkinson]

LGTM