[Refactor] [cuda] Cleanup CUDA AtomicOpStmt codegen (#2490)
[Refactor] [cuda] Cleanup CUDA AtomicOpStmt codegen (#2490)

* cuda codegen atomics cleanup

* clean up non-reduction atomics as well

* early return nullptr
yuanming-hu/taichi,yuanming-hu/taichi,yuanming-hu/taichi,yuanming-hu/taichi