[1/N][CustomOp] Register activation customop instead of overwrite forward_oot (#1841)

### What this PR does / why we need it?
We'll refator `CustomOp` in vllm-ascend from this pr on. 

Use function `CustomOp.register_oot` to achieve the customop registery,
taking `AscendQuickGELU` as an example:
```python
from vllm_ascend.ops.activation import AscendQuickGELU
CustomOp.register_oot(_decorated_op_cls=AscendQuickGELU, name="QuickGELU")
```

This is a quick adapt for `CustomOp.register_oot` mechanism from vllm
0.9.2. For further step, we can remove inherit from `QuickGELU` can
write our own `QuickGELU` at all.

Part of https://github.com/vllm-project/vllm-ascend/pull/1647



- vLLM version: v0.9.2
- vLLM main:
8dfb45ca33

---------

Signed-off-by: MengqingCao <cmq0113@163.com>

tests/ut/test_utils.py

@@ -301,6 +301,24 @@ class TestUtils(TestBase):
         self.assertFalse(utils.check_kv_cache_bytes_cache_exist(),
                          "Delete kv cache bytes cache dir failed")
 
+    @mock.patch("vllm.model_executor.custom_op.CustomOp")
+    @mock.patch("vllm_ascend.ops.activation.AscendQuickGELU")
+    @mock.patch("vllm_ascend.ops.activation.AscendSiluAndMul")
+    def test_register_ascend_customop(self, mock_ascend_silu_and_mul,
+                                      mock_ascend_quick_gelu, mock_customop):
+        utils._ASCEND_CUSTOMOP_IS_REIGISTERED = False
+
+        # ascend custom op is not registered
+        utils.register_ascend_customop()
+        # should call register_oot twice
+        self.assertEqual(mock_customop.register_oot.call_count, 2)
+        self.assertTrue(utils._ASCEND_CUSTOMOP_IS_REIGISTERED)
+
+        # ascend custom op is already registered
+        utils.register_ascend_customop()
+        # should not register_oot again, thus only called twice in this ut
+        self.assertEqual(mock_customop.register_oot.call_count, 2)
+
 
 class TestProfileExecuteDuration(unittest.TestCase):