[Misc] Add CustomOp interface for device portability (#5255)

tests/kernels/test_layernorm.py

@@ -42,7 +42,7 @@ def test_rms_norm(
 
     # NOTE(woosuk): The reference implementation should be executed first
     # because the custom kernel is in-place.
-    ref_out = layer._forward(x, residual)
+    ref_out = layer.forward_native(x, residual)
     out = layer(x, residual)
     # NOTE(woosuk): LayerNorm operators (including RMS) typically have larger
     # numerical errors than other operators because they involve reductions.