[v1] AttentionMetadata for each layer (#17394)

Signed-off-by: Chen Zhang <zhangch99@outlook.com>

vllm/v1/attention/backends/utils.py

@@ -0,0 +1,18 @@
+# SPDX-License-Identifier: Apache-2.0
+from dataclasses import dataclass
+
+import torch
+
+
+@dataclass
+class CommonAttentionMetadata:
+    """
+    Attention metadata attributes that can be shared by layers in different KV
+    cache groups and thus having different block table.
+    """
+
+    query_start_loc: torch.Tensor
+    """(batch_size + 1,), the start location of each request in query Tensor"""
+    seq_lens: torch.Tensor
+    """(batch_size,), the length of each request including both computed tokens
+    and newly scheduled tokens"""