Fix and move import statement to the top

Author: akihironitta

pytorch_lightning/utilities/distributed.py

@@ -25,6 +25,7 @@
 from pytorch_lightning.utilities.rank_zero import rank_zero_only  # noqa: F401
 from pytorch_lightning.utilities.rank_zero import rank_zero_deprecation
 from pytorch_lightning.utilities.rank_zero import rank_zero_info as new_rank_zero_info
+from pytorch_lightning.utilities.rank_zero import rank_zero_warn as new_rank_zero_warn
 
 if _TPU_AVAILABLE:
     import torch_xla.core.xla_model as xm
@@ -281,8 +282,6 @@ def register_ddp_comm_hook(
         ...     ddp_comm_wrapper=default.fp16_compress_wrapper,
         ... )
     """
-    from pytorch_lightning.utilities import rank_zero_warn
-
     if ddp_comm_hook is None:
         return
     # inform mypy that ddp_comm_hook is callable