[2/N][refactor] torchair deepseek mla backend refactor (#2459)

### What this PR does / why we need it?
This PR move current unified mla backend to torchair folder and remove
torchair-related code in attention/mla_v1.py (1.3k -> 0.9k).

 
### Does this PR introduce _any_ user-facing change?
No.
### How was this patch tested?
Running eager mode with mla backend, and torchair mode with code before
[2445](https://github.com/vllm-project/vllm-ascend/pull/2445)


- vLLM version: v0.10.0
- vLLM main:
f571ff8eb6

Signed-off-by: linfeng-yuan <1102311262@qq.com>

tests/ut/test_platform.py

@@ -425,6 +425,27 @@ class TestNPUPlatform(TestBase):
         self.assertEqual(result,
                          "vllm_ascend.attention.mla_v1.AscendMLABackend")
 
+    @patch('vllm_ascend.platform.get_ascend_config')
+    def test_get_attn_backend_cls_use_v1_mla_and_torchair(
+            self, mock_get_ascend_config):
+        mock_config = MagicMock()
+        mock_config.torchair_graph_config.enabled = True
+
+        mock_get_ascend_config.return_value = mock_config
+
+        result = self.platform.get_attn_backend_cls(
+            selected_backend="ascend",
+            head_size=64,
+            dtype="float16",
+            kv_cache_dtype="float16",
+            block_size=64,
+            use_v1=True,
+            use_mla=True,
+        )
+        self.assertEqual(
+            result,
+            "vllm_ascend.torchair.torchair_mla.AscendMLATorchairBackend")
+
     @patch('vllm_ascend.platform.get_ascend_config')
     def test_get_attn_backend_cls_use_v1_and_torchair(self,
                                                       mock_get_ascend_config):