[Kernel] Correctly invoke prefill & decode kernels for cross-attentio…

…n (towards eventual encoder/decoder model support) (vllm-project#4888)

Co-authored-by: Woosuk Kwon <woosuk.kwon@berkeley.edu>
Signed-off-by: Alvant <alvasian@yandex.ru>

tests/kernels/test_attention_selector.py

@@ -47,32 +47,32 @@ def test_flash_attn(monkeypatch):
     # Unsupported CUDA arch
     with patch("torch.cuda.get_device_capability", return_value=[7, 5]):
         backend = which_attn_to_use(8, 16, 8, None, torch.float16, None, 16)
-        assert backend.name != "FLASH_ATTN"
+        assert backend.name != STR_FLASH_ATTN_VAL
 
     # Unsupported data type
     backend = which_attn_to_use(8, 16, 8, None, torch.float8_e4m3fn, None, 16)
-    assert backend.name != "FLASH_ATTN"
+    assert backend.name != STR_FLASH_ATTN_VAL
 
     # Unsupported kv cache data type
     backend = which_attn_to_use(8, 16, 8, None, torch.float16, "fp8", 16)
-    assert backend.name != "FLASH_ATTN"
+    assert backend.name != STR_FLASH_ATTN_VAL
 
     # Unsupported block size
     backend = which_attn_to_use(8, 16, 8, None, torch.float16, None, 8)
-    assert backend.name != "FLASH_ATTN"
+    assert backend.name != STR_FLASH_ATTN_VAL
 
     # Unsupported sliding window
     backend = which_attn_to_use(8, 16, 8, 1, torch.float16, None, 16)
-    assert backend.name != "FLASH_ATTN"
+    assert backend.name != STR_FLASH_ATTN_VAL
 
     # flash-attn is not installed
     with patch.dict('sys.modules', {'vllm_flash_attn': None}):
         backend = which_attn_to_use(8, 16, 8, None, torch.float16, None, 16)
-        assert backend.name != "FLASH_ATTN"
+        assert backend.name != STR_FLASH_ATTN_VAL
 
     # Unsupported head size
     backend = which_attn_to_use(8, 17, 8, None, torch.float16, None, 16)
-    assert backend.name != "FLASH_ATTN"
+    assert backend.name != STR_FLASH_ATTN_VAL
 
 
 def test_invalid_env(monkeypatch):