[CustomOp] Register RMSNorm instead of overwrite forward_oot (#2284)

### What this PR does / why we need it?
Use function CustomOp.register_oot to achieve the customop registery
```
from vllm.model_executor.custom_op import CustomOp
CustomOp.register_oot(_decorated_op_cls=AscendRMSNorm, name="RMSNorm")
```

### Does this PR introduce _any_ user-facing change?
N/A

### How was this patch tested?
CI passed with new added/existing test.

- vLLM version: v0.10.0
- vLLM main:
afa5b7ca0b

---------

Signed-off-by: Icey <1790571317@qq.com>

vllm_ascend/utils.py

@@ -479,6 +479,9 @@ def register_ascend_customop():
     CustomOp.register_oot(_decorated_op_cls=AscendSiluAndMul,
                           name="SiluAndMul")
 
+    from vllm_ascend.ops.layernorm import AscendRMSNorm
+    CustomOp.register_oot(_decorated_op_cls=AscendRMSNorm, name="RMSNorm")
+
     # NOTE: Keep this at last to ensure all custom actions are registered
     _ASCEND_CUSTOMOP_IS_REIGISTERED = True