[RoadMap] Multi-Backend Refactoring/Decoupling

@SiriusNEO @LeiWang1999

TileLang currently supports multiple backends (CUDA, ROCm, etc.), but the codebase has become deeply coupled with specific backends. This tight coupling makes it difficult to add new backends and maintain existing ones.

This roadmap tracks our recent efforts on this refactoring. We will gradually refactor and reorganize the codebase to reduce backend coupling, establishing clear abstractions and interfaces that make it easy to integrate new backends.

- [x] https://github.com/tile-ai/tilelang/pull/2114/
- [x] https://github.com/tile-ai/tilelang/pull/2121/
- [x] https://github.com/tile-ai/tilelang/pull/2138/
- [x] https://github.com/tile-ai/tilelang/pull/2189/
- [x] https://github.com/tile-ai/tilelang/pull/2291
- [x] https://github.com/tile-ai/tilelang/pull/2297
- [x] https://github.com/tile-ai/tilelang/pull/2323
- [x] https://github.com/tile-ai/tilelang/pull/2442
- [x] https://github.com/tile-ai/tilelang/pull/2446
- [x] https://github.com/tile-ai/tilelang/pull/2734
- [x] https://github.com/tile-ai/tilelang/pull/2870/
- [x] https://github.com/tile-ai/tilelang/pull/2318/
- [x] https://github.com/tile-ai/tilelang/pull/2900