[3/N][Refactor][Qwen3-Next] Refacotr model structure and fix bug by vllm #25400 (#3142)

### What this PR does / why we need it?
Refactor model structure in qwen3_next.py to reduce code line.

### Does this PR introduce _any_ user-facing change?
N/A

### How was this patch tested?
```
def main():
    prompts = [
        "The future of AI is",
    ]

    # Create a sampling params object.
    sampling_params = SamplingParams(max_tokens=100, temperature=0.6, top_k=40, top_p=0.95)
    # Create an LLM.
    llm = LLM(
        model="Qwen/Qwen3-Next-80B-A3B-Instruct",
              tensor_parallel_size=4,
              enforce_eager=True,
              trust_remote_code=True,
              max_model_len=256,
              gpu_memory_utilization=0.7,
              block_size=64,
              )
    # Generate texts from the prompts.
    outputs = llm.generate(prompts, sampling_params)
    for output in outputs:
        prompt = output.prompt
        generated_text = output.outputs[0].text
        print(f"Prompt: {prompt!r}, Generated text: {generated_text!r}")
```


- vLLM version: v0.10.2
- vLLM main:
vllm-project/vllm@releases/v0.11.0

---------

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

Author: wxsIcey

vllm_ascend/models/__init__.py

@@ -53,4 +53,4 @@ def register_model():
     )
     ModelRegistry.register_model(
         "Qwen3NextForCausalLM",
-        "vllm_ascend.models.qwen3_next:Qwen3NextForCausalLM")
+        "vllm_ascend.models.qwen3_next:CustomQwen3NextForCausalLM")