[refactor] refactor model runner capture model (#5230)

### What this PR does / why we need it?
Refactor the `capture_model` method in model_runner to directly reuse
the method from vLLM.

Currently, most of the logic in the capture_model method is similar to
that in the vllm code. Directly using the vllm method can reduce the
maintenance cost of the vllm-ascend code. Modify as follows:
1、refactor capture_model function, directly inheriting community methods
2、refactor initialize_aclgraph_capture function, move to
initialize_attn_backend

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?

- vLLM version: release/v0.13.0
- vLLM main:
ad32e3e19c

Signed-off-by: weiguihua2 <weiguihua2@huawei.com>
Co-authored-by: weijinqian0 <1184188277@qq.com>

tests/e2e/singlecard/test_aclgraph_mem.py

@@ -54,10 +54,10 @@ def test_aclgraph_mem_use(model: str, max_tokens: int) -> None:
 
         return wrapped
 
-    original_capture = NPUModelRunner._capture_model
+    original_capture = NPUModelRunner.capture_model
 
     with patch.object(NPUModelRunner,
-                      '_capture_model',
+                      'capture_model',
                       new=capture_model_wrapper(original_capture)):
         prompts = [
             "Hello, my name is", "The president of the United States is",
@@ -73,7 +73,7 @@ def test_aclgraph_mem_use(model: str, max_tokens: int) -> None:
             vllm_model = VllmRunner(snapshot_download(model))
         _ = vllm_model.generate(prompts, sampling_params)
 
-    assert capture_called.value == 1, "_capture_model was not called during test"
+    assert capture_called.value == 1, "capture_model was not called during test"
     assert capture_mem_before.value != -1, "capture_mem_before not set"
     assert capture_mem_after.value != -1, "capture_mem_after not set"
 
@@ -93,7 +93,7 @@ def test_aclgraph_mem_use(model: str, max_tokens: int) -> None:
     max_capture_mem_gib = baseline_capture_mem * capture_mem_tolerance
     max_mem_expected = max_capture_mem_gib * (1024**3)
     assert mem_used_by_capture < max_mem_expected, (
-        f"_capture_model used more memory than expected. "
+        f"capture_model used more memory than expected. "
         f"Used: {mem_used_by_capture / (1024**3):.2f} GiB, "
         f"Expected: < {max_capture_mem_gib:.2f} GiB")
     os.environ["VLLM_WORKER_MULTIPROC_METHOD"] = 'spawn'