[npugraph_ex]enable npugraph_ex by default (#6664)

### What this PR does / why we need it?

This pull request enables the `npugraph_ex` backend by default to
improve performance on Ascend NPUs, as proposed in the
[RFC](https://github.com/vllm-project/vllm-ascend/issues/6214).


### Does this PR introduce _any_ user-facing change?

Yes. `npugraph_ex` is now enabled by default. Users can disable it by
setting `enable: false` in the `npugraph_ex_config` section of the
`additional_config`.

### How was this patch tested?

CI passed. The changes are covered by existing and new E2E tests
(`test_aclgraph_accuracy.py`) and unit tests (`test_ascend_config.py`)
that have been updated to reflect the new default behavior. The tests
verify correctness and consistency with `npugraph_ex` enabled and
disabled, as well as with the new static kernel option.

Signed-off-by: huyuanquan1 <huyuanquan1@huawei.com>
Co-authored-by: huyuanquan1 <huyuanquan1@huawei.com>

vllm_ascend/ascend_config.py

@@ -260,7 +260,7 @@ class NpugraphExConfig:
 
     def __init__(
         self,
-        enable: bool = False,
+        enable: bool = True,
         enable_static_kernel: bool = False,
         fuse_norm_quant: bool = True,
         fuse_qknorm_rope: bool = True,
@@ -274,7 +274,7 @@ class NpugraphExConfig:
             enable (bool): Whether to enable npugraph_ex backend.
                 When set to True, the Fx graph generated by Dymano will be
                 optimized and compiled by the npugraph_ex backend.
-                Default: False
+                Default: True
             enable_static_kernel (bool): Whether to enable static kernel.
                 Static kernel is suitable for scenarios with purely static shapes
                 or minimal shape changes, and can improve network performance.