* [Backend] Refactor Pipeline to support different backends
Introduce Pipeline abstraction in backend/pipeline.py with per-backend
registration. Each backend (cuda, hip, c, llvm) now registers its own
compilation pass pipeline. engine/lower.py resolves the pipeline from
the target instead of hardcoding phase imports.
* enhance the structure
* fuse two stage
* fix after rebase
* update template pass pipeline
* pipeline
* fix test
* fix test
* fix cuda pipeline prologue