Fix unrecognized local rank argument (#676)

train.py

@@ -433,7 +433,7 @@ if __name__ == '__main__':
     parser.add_argument('--single-cls', action='store_true', help='train as single-class dataset')
     parser.add_argument('--adam', action='store_true', help='use torch.optim.Adam() optimizer')
     parser.add_argument('--sync-bn', action='store_true', help='use SyncBatchNorm, only available in DDP mode')
-    parser.add_argument('--local-rank', type=int, default=-1, help='DDP parameter, do not modify')
+    parser.add_argument('--local_rank', type=int, default=-1, help='DDP parameter, do not modify')
     parser.add_argument('--logdir', type=str, default='runs/', help='logging directory')
     opt = parser.parse_args()