Makes HfArgumentParser compatible with Python 3.9 (#9479)

Python 3.9 changed the format of the string serialization of `typing.Optional`.
For example, `str(typing.Optional[str])` is
`typing.Union[str, NoneType]` in python 3.8 and
`typing.Optional[str]` in Python 3.9.

src/transformers/hf_argparser.py

@@ -66,9 +66,15 @@ class HfArgumentParser(ArgumentParser):
             typestring = str(field.type)
             for prim_type in (int, float, str):
                 for collection in (List,):
-                    if typestring == f"typing.Union[{collection[prim_type]}, NoneType]":
+                    if (
+                        typestring == f"typing.Union[{collection[prim_type]}, NoneType]"
+                        or typestring == f"typing.Optional[{collection[prim_type]}]"
+                    ):
                         field.type = collection[prim_type]
-                if typestring == f"typing.Union[{prim_type.__name__}, NoneType]":
+                if (
+                    typestring == f"typing.Union[{prim_type.__name__}, NoneType]"
+                    or typestring == f"typing.Optional[{prim_type.__name__}]"
+                ):
                     field.type = prim_type
 
             if isinstance(field.type, type) and issubclass(field.type, Enum):