feat: add retireval_top_n to config in env

[ProseGuys]

Summary

Solution: Add a new retrieval-related parameter top_n in the environment variables. During the retrieval process, use the default top_k parameter to obtain the top_k most relevant slices. When reranking, use the top_n parameter to return the top_n slices.

Tip

Close issue syntax: Fixes #<issue number> or Resolves #<issue number>, see documentation for more details.
Resolves #11068

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Checklist

Important

Please review the checklist below before submitting your pull request.

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[nadirvishun]

Currently, in my own project, I am also using similar environment variables to simply distinguish between topK and topN. However, a better approach would be to differentiate them on the knowledge base configuration interface (and also add a threshold parameter), rather than handling it through coding.

[ProseGuys]

Currently, in my own project, I am also using similar environment variables to simply distinguish between topK and topN. However, a better approach would be to differentiate them on the knowledge base configuration interface (and also add a threshold parameter), rather than handling it through coding.目前，在我自己的项目中，我也在使用类似的环境变量来简单区分topK和topN 。然而，更好的方法是在知识库配置界面上区分它们（并添加threshold参数），而不是通过编码来处理。

Yes, this is only a temporary solution for users with a rag background. Adding relevant parameters on the page side would involve product design, so we have not made any relevant modifications.

[laipz8200]

This is the wrong way to use the config object.

[ProseGuys]

Sorry, I made a stupid mistake and caused a bad impact on your, but the revised submission

[laipz8200]

@ProseGuys We reverted this change in this version and reopened the related issue.