auto port kill before starting ddp

pytorch_lightning/models/trainer.py

@@ -421,6 +421,7 @@ class Trainer(TrainerIO):
 
         # when using multi-node or DDP within a node start each module in a separate process
         if self.use_ddp:
+            # clear any processes running on the ddp port
             self.__kill_ddp_ports(self.ddp_port)
 
             # must copy only the meta of the exp so it survives pickle/unpickle when going to new process