[bugfix] fix pass bug: pass really rope dim for npu_rotary_embedding (#6880)

### What this PR does / why we need it?
pass really rope dim for npu_rotary_embedding
**before：**
            q_rope, k_rope = torch.ops.vllm.npu_rotary_embedding(
positions, q_flat, k_flat, cos_sin_cache, self.head_dim,
**self.head_dim,** True
            )
**after：**
            q_rope, k_rope = torch.ops.vllm.npu_rotary_embedding(
positions, q_flat, k_flat, cos_sin_cache, self.head_dim,
**self.rope_dim,** True
            )
### Does this PR introduce _any_ user-facing change?
no
### How was this patch tested?

- vLLM version: v0.16.0
- vLLM main:
15d76f74e2

---------

Signed-off-by: zjks98 <zhangjiakang4@huawei.com>
Signed-off-by: aipaes <82140963+aipaes@users.noreply.github.com>
Co-authored-by: zjks98 <zhangjiakang4@huawei.com>

vllm_ascend/utils.py

@@ -1185,3 +1185,19 @@ def check_gdn_layer(vllm_config) -> bool:
             return True
 
     return False
+
+
+def get_rope_dim(vllm_config):
+    model_config = vllm_config.model_config
+
+    if model_config.use_mla:
+        rope_dim = model_config.hf_text_config.qk_rope_head_dim
+    else:
+        rope_dim = model_config.get_head_size()
+        # For models using partial rope like Qwen3-Next.
+        if hasattr(model_config.hf_text_config, "partial_rotary_factor"):
+            rope_dim = int(rope_dim * model_config.hf_text_config.partial_rotary_factor)
+        elif hasattr(model_config.hf_text_config, "rotary_dim"):
+            rope_dim = int(model_config.hf_text_config.rotary_dim)
+
+    return rope_dim