Fix type of c10d bucket size

fairseq/options.py

@@ -187,7 +187,7 @@ def add_distributed_training_args(parser):
                        help='which GPU to use (usually configured automatically)')
     group.add_argument('--no-c10d', action='store_true',
                        help='don\'t use c10d distributed backend')
-    group.add_argument('--c10d-bucket-cap-mb', default=150, metavar='MB',
+    group.add_argument('--c10d-bucket-cap-mb', default=150, type=int, metavar='MB',
                        help='bucket size for c10d backend')
     return group