[Encoder Decoder] Add flash_attn kernel support for encoder-decoder models (#9559)

vllm/attention/selector.py

@@ -98,7 +98,6 @@ def get_attn_backend(
     is_blocksparse: bool = False,
 ) -> Type[AttentionBackend]:
     """Selects which attention backend to use and lazily imports it."""
-
     if is_blocksparse:
         logger.info("Using BlocksparseFlashAttention backend.")
         from vllm.attention.backends.blocksparse_attn import (
@@ -108,6 +107,7 @@ def get_attn_backend(
     backend = which_attn_to_use(head_size, dtype, kv_cache_dtype, block_size,
                                 is_attention_free)
     if backend == _Backend.FLASH_ATTN:
+        logger.info("Using Flash Attention backend.")
         from vllm.attention.backends.flash_attn import (  # noqa: F401
             FlashAttentionBackend)
         return FlashAttentionBackend