[Misc] Fix docstring of get_attn_backend (vllm-project#5271)

vllm/attention/selector.py

@@ -31,15 +31,14 @@ def get_attn_backend(
     block_size: int,
     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 (
             BlocksparseFlashAttentionBackend)
         return BlocksparseFlashAttentionBackend
-    """Determine which attention backend to use and only import
-    the selected backend module.
-    """
+
     backend = which_attn_to_use(num_heads, head_size, num_kv_heads,
                                 sliding_window, dtype, kv_cache_dtype,
                                 block_size)