[Kernel] Move attn_type to Attention.__init__() (#11690)

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

tests/kernels/utils.py

@@ -13,6 +13,7 @@ from torch._prims_common import TensorLikeType
 
 from vllm.attention import AttentionBackend, AttentionMetadata, AttentionType
 from vllm.model_executor.layers.activation import SiluAndMul
+from vllm.platforms.interface import _Backend
 from vllm.utils import (STR_BACKEND_ENV_VAR, STR_FLASH_ATTN_VAL,
                         STR_XFORMERS_ATTN_VAL, make_tensor_with_pad)
 
@@ -790,7 +791,7 @@ def make_block_tables_slot_mapping(
 
 
 def make_test_metadata(
-    attn_backend: AttentionBackend,
+    attn_backend: _Backend,
     is_prompt: bool,
     seq_lens: Optional[List[int]],
     decoder_test_params: Optional[PhaseTestParameters],
@@ -815,7 +816,7 @@ def make_test_metadata(
 
     Arguments:
 
-    * attn_backend: Backend for sourcing attention kernels
+    * attn_backend_name: Backend for sourcing attention kernels
     * is_prompt: prefill if True, o/w decode
     * seq_lens: list of token counts for each sequence
     * decoder_test_params: decoder self-attention test params; 
@@ -882,6 +883,8 @@ def make_test_metadata(
         #   (kv_mmap)
         cross_kv_mmap = cross_test_params.kv_mmap
 
+    attn_backend_obj = make_backend(attn_backend.name)
+
     if is_prompt:
         # Prefill-phase scenario
 
@@ -902,8 +905,7 @@ def make_test_metadata(
                                    context_lens,
                                    encoder_seq_lens,
                                    device=device)
-
-        return attn_backend.make_metadata(
+        return attn_backend_obj.make_metadata(
             num_prefills=num_prefills,
             slot_mapping=(None if kv_mmap is None else kv_mmap.slot_mapping),
             multi_modal_placeholder_index_maps=None,
@@ -952,7 +954,7 @@ def make_test_metadata(
                                    encoder_seq_lens,
                                    device=device)
 
-        return attn_backend.make_metadata(
+        return attn_backend_obj.make_metadata(
             num_prefills=num_prefills,
             slot_mapping=kv_mmap.slot_mapping,
             multi_modal_placeholder_index_maps=None,