[Feature]cpu offload connector (#1659)

This PR implements cpu offload connector to enable NPU kv cache offload
to host DRAM.

- vLLM version: v0.10.2
- vLLM main:
5aeb925452

Signed-off-by: lidenghui <lidenghui1110@gmail.com>
Signed-off-by: AlvisGong <gwly0401@163.com>
Signed-off-by: CalvinXKY <kyxiezju@163.com>
Co-authored-by: AlvisGong <gwly0401@163.com>

vllm_ascend/attention/utils.py

@@ -1,7 +1,11 @@
 from dataclasses import dataclass
-from typing import Any
+from typing import Any, List
 
 import torch
+from vllm.distributed.kv_transfer import (get_kv_transfer_group,
+                                          has_kv_transfer_group,
+                                          is_v1_kv_transfer_group)
+from vllm.forward_context import ForwardContext, get_forward_context
 
 
 @dataclass
@@ -100,3 +104,34 @@ def split_decodes_and_prefills(
     num_decode_tokens = query_start_loc[first_prefill].item()
     num_prefill_tokens = num_tokens - num_decode_tokens
     return (num_decodes, num_prefills, num_decode_tokens, num_prefill_tokens)
+
+
+def wait_for_kv_layer_from_connector(layer_name: str):
+    if not has_kv_transfer_group() or not is_v1_kv_transfer_group():
+        return
+
+    connector = get_kv_transfer_group()
+
+    forward_context: ForwardContext = get_forward_context()
+    attn_metadata = forward_context.attn_metadata
+    if attn_metadata is None:
+        return
+    # TODO: assert ascendMetadata
+    connector.wait_for_layer_load(layer_name)
+
+
+def maybe_save_kv_layer_to_connector(
+    layer_name: str,
+    kv_cache_layer: List[torch.Tensor],
+):
+    if not has_kv_transfer_group() or not is_v1_kv_transfer_group():
+        return
+
+    connector = get_kv_transfer_group()
+
+    forward_context: ForwardContext = get_forward_context()
+    attn_metadata = forward_context.attn_metadata
+    if attn_metadata is None:
+        return
+    # TODO: assert ascendMetadata
+    connector.save_kv_layer(layer_name, kv_cache_layer, attn_metadata)