[Misc] Fix flash attention backend log (#4368)

Author: esmeetu

vllm/attention/selector.py

@@ -25,7 +25,7 @@ class _Backend(enum.Enum):
 def get_attn_backend(dtype: torch.dtype) -> Type[AttentionBackend]:
     backend = _which_attn_to_use(dtype)
     if backend == _Backend.FLASH_ATTN:
-        logger.info("Using FlashAttention backend.")
+        logger.info("Using FlashAttention-2 backend.")
         from vllm.attention.backends.flash_attn import (  # noqa: F401
             FlashAttentionBackend)
         return FlashAttentionBackend
@@ -62,21 +62,21 @@ def _which_attn_to_use(dtype: torch.dtype) -> _Backend:
     # NVIDIA GPUs.
     if torch.cuda.get_device_capability()[0] < 8:
         # Volta and Turing NVIDIA GPUs.
-        logger.info("Cannot use FlashAttention backend for Volta and Turing "
+        logger.info("Cannot use FlashAttention-2 backend for Volta and Turing "
                     "GPUs.")
         return _Backend.XFORMERS
 
     if dtype not in (torch.float16, torch.bfloat16):
-        logger.info("Cannot use FlashAttention backend for dtype other than "
+        logger.info("Cannot use FlashAttention-2 backend for dtype other than "
                     "torch.float16 or torch.bfloat16.")
         return _Backend.XFORMERS
 
     try:
         import flash_attn  # noqa: F401
     except ImportError:
         logger.info(
-            "Cannot use FlashAttention backend because the flash_attn package "
-            "is not found. Please install it for better performance.")
+            "Cannot use FlashAttention-2 backend because the flash_attn "
+            "package is not found. Please install it for better performance.")
         return _Backend.XFORMERS
 
     backend_by_env_var = os.getenv(VLLM_ATTENTION_BACKEND)