fix the convert.py args (#556)

x2paddle/convert.py

@@ -41,7 +41,6 @@ def arg_parser():
     parser.add_argument(
         "--save_dir",
         "-s",
-        required=True,
         type=_text_type,
         default=None,
         help="path to save translated model")
@@ -221,6 +220,8 @@ def main():
             x2paddle.__version__))
         return
 
+    assert args.save_dir is not None, "--save_dir is not defined"
+
     try:
         import platform
         v0, v1, v2 = platform.python_version().split('.')