[Platform] Add import_kernels interface (#3694)

### What this PR does / why we need it?
Add import_kernels interface to avoid import useless vLLM C library

Closes #3488. Reopen #3498 for CI.

### How was this patch tested?

CI tested.

- vLLM version: v0.11.0
- vLLM main:
vllm-project/vllm@2918c1b

---------

Signed-off-by: gcanlin <[email protected]>

Author: gcanlin

vllm_ascend/platform.py

@@ -370,6 +370,17 @@ def check_and_update_config(cls, vllm_config: VllmConfig) -> None:
                 "needs to be equal if use cp or dcp > 1 in P/D disaggregate scenario."
             )
 
+    @classmethod
+    def import_kernels(cls) -> None:
+        # Directly importing vllm_ascend_C prevents ASCEND_RT_VISIBLE_DEVICES
+        # from being applied during runtime initialization, which causes bugs
+        # in the RL module. Therefore, we currently use lazy initialization
+        # to avoid this issue. See https://github.com/vllm-project/vllm-ascend/pull/884.
+        # TODO: when the above issue is fixed, we can uncomment the following lines.
+        # from vllm_ascend.utils import enable_custom_op
+        # enable_custom_op()
+        pass
+
     @classmethod
     def get_attn_backend_cls(
         cls,