chore: allow custom paths for external dependencies like CUTLASS

[yzh119]

📌 Description

Implement features mentioned in #1806 , now user can specify custom dependencies path (e.g. if they are installed as system library) via environment variables FLASHINFER_CUTLASS_INCLUDE_PATH etc.

User can run python -m flashinfer show-config to display these environment variables.

cc @bwl1289

🔍 Related Issues

#1806

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Reviewer Notes

[gemini-code-assist[bot]]

Summary of Changes

Hello @yzh119, I'm Gemini Code Assist[^1]! 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 enhances the flexibility of FlashInfer's build system by allowing users to define custom installation paths for critical external dependencies such as CUTLASS, SPDLOG, and NVSHMEM. This change simplifies integration into diverse development environments and system configurations, reducing potential conflicts and improving overall usability for advanced users who manage their own dependency installations. The update also ensures that configuration details are easily inspectable via the show-config utility.

Highlights

• Custom Dependency Paths: Introduced the ability for users to specify custom paths for external dependencies like CUTLASS, SPDLOG, and NVSHMEM using new environment variables (e.g., FLASHINFER_CUTLASS_INCLUDE_PATH).
• Backward Compatibility: Ensured backward compatibility for NVSHMEM-related environment variables by checking for the new FLASHINFER_ prefixed variables first, then falling back to the older NVSHMEM_ prefixed ones.
• Configuration Visibility: Updated the python -m flashinfer show-config command to display the values of these new environment variables, providing better transparency for users.
• Dynamic Path Resolution: Refactored the internal handling of CUTLASS and SPDLOG include paths to use getter functions, allowing dynamic resolution based on environment variables rather than static paths.

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[zhyncs]

@yzh119 even we don't want to release rc version, we need to fix the release issue https://github.com/flashinfer-ai/flashinfer/actions/runs/18338855616/job/52229146277