fix horovod

Author: four4fish

pytorch_lightning/trainer/connectors/accelerator_connector.py

@@ -75,6 +75,9 @@
 
 log = logging.getLogger(__name__)
 
+if _HOROVOD_AVAILABLE:
+    import horovod.torch as hvd
+
 
 class AcceleratorConnector:
     def __init__(
@@ -562,8 +565,6 @@ def handle_horovod(self):
                 "Install with \n $HOROVOD_WITH_PYTORCH=1 pip install horovod[pytorch]"
             )
 
-        import horovod.torch as hvd
-
         hvd.init()
         if isinstance(self.accelerator, GPUAccelerator):
             # Horovod assigns one local GPU per process