## Summary
Backend loading is easier to maintain when native backend library
discovery, in-tree backend Python hook loading, and out-of-tree entry
point autoload are owned by the backend namespace. This PR consolidates
those paths under `tvm.backend._autoload_backends` while preserving
compatibility routes from the previous top-level helper and
`tvm.base.load_backend_libs`.
- Move backend runtime DSO loading into `tvm.backend._autoload_backends`
- Route `backend.load_all()` through the backend autoload helper
- Keep the previous top-level `_autoload_backends` module as a thin
compatibility import
Tianqi Chen