[4/N][Refactor] torchair model runner refactor (#2208)

There is lot of torchair code in model runner leading the code hard for
maintenance. We'll create new torchair_model_runner to split torchair
related logic. Following the workflow #2203, this is the first PR.

What's this PR do:

create common function `_convert_torch_foramt`  for initialize_kv_cache


- vLLM version: v0.10.0
- vLLM main:
14a5d903ab

Signed-off-by: wangxiyuan <wangxiyuan1007@gmail.com>

vllm_ascend/torchair/torchair_model_runner.py

@@ -20,10 +20,11 @@
 from typing import Optional
 
 import torch
+import torch_npu
 from vllm.config import VllmConfig
 from vllm.forward_context import get_forward_context
 
-from vllm_ascend.utils import (ACL_FORMAT_FRACTAL_NZ,
+from vllm_ascend.utils import (ACL_FORMAT_FRACTAL_ND, ACL_FORMAT_FRACTAL_NZ,
                                maybe_converting_weight_acl_format)
 from vllm_ascend.worker.model_runner_v1 import NPUModelRunner
 
@@ -113,3 +114,7 @@ class NPUTorchairModelRunner(NPUModelRunner):
                 with_prefill, is_torchair_compile, input_ids, positions,
                 attn_metadata, num_tokens, intermediate_tensors, inputs_embeds)
         return hidden_states
+
+    def _convert_torch_format(self, kv_cache):
+        kv_cache = torch_npu.npu_format_cast(kv_cache, ACL_FORMAT_FRACTAL_ND)
+        return kv_cache