enable npugraph_ex (#5120)

### What this PR does / why we need it?
We will expose the enabling switch for npugraph_ex to better facilitate
subsequent optimization.

### Does this PR introduce _any_ user-facing change?
Previously, the enable_npugraph_ex switch would trigger an error; now we
have removed the error reporting mechanism to better facilitate
subsequent optimization efforts.
Basic functionalities are available in CANN and torch_npu for Q3, while
advanced optimizations will depend on the Q4 release.

### How was this patch tested?
llm =LLM(
    model=model,
    enforce_eager=False ,
        additional_config={
        "enable_npugraph_ex":  True
        },
        compilation_config={
            "cudagraph_mode": "FULL_DECODE_ONLY",
            "cudagraph_capture_sizes": [16],
        },
}


- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c

---------

Signed-off-by: p00465316 <panchao13@huawei.com>
Co-authored-by: p00465316 <panchao13@huawei.com>
Co-authored-by: weijinqian0 <1184188277@qq.com>

vllm_ascend/ascend_config.py

@@ -150,10 +150,6 @@ class AscendConfig:
             self, vllm_config)
         self.enable_npugraph_ex = additional_config.get(
             "enable_npugraph_ex", False)
-        if self.enable_npugraph_ex:
-            raise NotImplementedError(
-                "This feature is still in the experiment and will be supported soon."
-            )
         # We find that _npu_paged_attention still performs better than
         # npu_fused_infer_attention_score in some cases. We allow to execute
         # _npu_paged_attention in this cases. This should be removed once