[3/N][Refactor] Move torchair_attention to torchair dir (#2017)

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

1. Move `torchair_attention` to `torchair` dir.
2. Make `AscendAttentionTorchairBackend` extend `AscendAttentionBackend`
to reduce duplicate methods.
3. Make `AscendTorchairMetadata` extend `AscendMetadata` to reduce
duplicate properties.

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

### How was this patch tested?


- vLLM version: v0.10.0
- vLLM main:
0933f9d518

---------

Signed-off-by: shen-shanshan <467638484@qq.com>

tests/ut/test_platform.py

@@ -444,7 +444,7 @@ class TestNPUPlatform(TestBase):
         )
         self.assertEqual(
             result,
-            "vllm_ascend.attention.attention_v1_torchair.AscendAttentionTorchairBackend"
+            "vllm_ascend.torchair.torchair_attention.AscendAttentionTorchairBackend"
         )
 
     @patch('vllm_ascend.platform.get_ascend_config')