[v0.18.0][BugFix] fix the weightsmapper bug of qwen3-vl (#7868)

### What this PR does / why we need it?
This PR fixes a weight loading error in the Qwen3-VL model.
The bug was introduced by vLLM. In vLLM's `qwen3-vl.py`, the prefix of
the `lm_head` layer is hardcoded as `"lm_head"`. However,
`hf_to_vllm_mapper` remaps the weight name of `lm_head` from `lm_head`
to `language_model.lm_head`.
This causes a mismatch between the keys in the weight file and the
prefix of the lm_head layer, resulting in an error.
### Does this PR introduce _any_ user-facing change?
### How was this patch tested?
- [x] Run Qwen3-VL dense model with the fusion operator, verify correct
output

Signed-off-by: betta18 <jiangmengyu1@huawei.com>
Co-authored-by: betta18 <jiangmengyu1@huawei.com>

vllm_ascend/quantization/modelslim_config.py

@@ -455,6 +455,10 @@ class AscendModelSlimConfig(QuantizationConfig):
                     parts = parts[: exp_idx + 1]
                     prefix = ".".join(parts)
 
+        # TODO: remove it when vllm fixes the WeightsMapper bug of qwen3-vl.
+        if model_type in ["qwen3_vl"] and prefix == "lm_head":
+            prefix = "language_model.lm_head"
+
         if model_type in packed_modules_model_mapping:
             self.packed_modules_mapping = packed_modules_model_mapping[model_type]
         prefix = self.quant_prefix_mapper(model_type, prefix)