[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>

vllm_ascend/platform.py

@@ -29,7 +29,7 @@ from vllm.platforms import Platform, PlatformEnum
 from vllm_ascend.ascend_config import (check_ascend_config, get_ascend_config,
                                        init_ascend_config)
 from vllm_ascend.utils import (ASCEND_QUATIZATION_METHOD, is_310p,
-                               update_aclgraph_sizes)
+                               register_ascend_customop, update_aclgraph_sizes)
 
 if TYPE_CHECKING:
     from vllm.config import ModelConfig, VllmConfig
@@ -205,6 +205,9 @@ class NPUPlatform(Platform):
                 ascend_config.ascend_scheduler_config)
             vllm_config.scheduler_config = ascend_scheduler_config
 
+        # register Ascend CustomOp
+        register_ascend_customop()
+
     @classmethod
     def get_attn_backend_cls(cls, selected_backend, head_size, dtype,
                              kv_cache_dtype, block_size, use_v1, use_mla):