## 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
This PR batches several post-bringup TIRx follow-ups, rebased onto
current `main`.
### Changes
- **op-dispatch**: per-call exec scope via `Tx.<scope>.op`; remove
`ExecScopeStmt`
- **namespaces**: split TIRx op namespaces; remove tile-primitive kind
attrs
- **codegen**: support explicit CUDA launch bounds
- **gemm-async**: support contiguous-axis (K-major) operand slicing
- **backend reorg**: move in-tree GPU backends out of core into
`src/backend/<target>/` and `python/tvm/backend/<target>/`
(codegen/runtime/op), with the corresponding `CMakeLists.txt` /
`cmake/modules` and include-path updates
### Testing
- Builds with `USE_CUDA=ON` / `USE_LLVM=ON`
- The TIRx Python test suite (`tests/python/tirx/`) passes locally
Tianqi Chen