Fixed type checking errors in node.py (pytorch#68124)

Summary:
Fixes [issue#67](MLH-Fellowship/pyre-check#67)
This PR fixes the type checking errors in Pytorch torch/fx/node.py .
The variable types in 363:20 and 364:20 were declared to have type `List[str]`  but were  assigned a value of  `None`. This caused an incompatitble variable type error.  I changed the type from `List[str]` to `Optional[List[str]` . This therefore fixed the incompatitble variable type error.

Signed-off-by: Onyemowo  Agbo
onionymous
0xedward

Pull Request resolved: pytorch#68124

Reviewed By: gmagogsfm

Differential Revision: D32322414

Pulled By: onionymous

fbshipit-source-id: be11bbbd463715ddf28a5ba78fb4adbf62878c80

Author: Onyiee

test/expect/TestFXAPIBackwardCompatibility.test_function_back_compat-fx_backcompat_function_signatures.expect

@@ -51,7 +51,7 @@ torch.fx.interpreter.Transformer.placeholder(self, target: 'Target', args: Tuple
 torch.fx.interpreter.Transformer.transform(self) -> torch.fx.graph_module.GraphModule
 torch.fx.node.Node.__init__(self, graph: 'Graph', name: str, op: str, target: 'Target', args: Tuple[Argument, ...], kwargs: Dict[str, Argument], return_type: Optional[Any] = None) -> None
 torch.fx.node.Node.append(self, x: 'Node') -> None
-torch.fx.node.Node.format_node(self, placeholder_names: List[str] = None, maybe_return_typename: List[str] = None) -> Optional[str]
+torch.fx.node.Node.format_node(self, placeholder_names: Optional[List[str]] = None, maybe_return_typename: Optional[List[str]] = None) -> Optional[str]
 torch.fx.node.Node.prepend(self, x: 'Node') -> None
 torch.fx.node.Node.replace_all_uses_with(self, replace_with: 'Node') -> List[Node]
 torch.fx.node.Node.replace_input_with(self, old_input: 'Node', new_input: 'Node')

torch/fx/node.py

@@ -407,8 +407,8 @@ def _pretty_print_target(self, target):
 
     @compatibility(is_backward_compatible=True)
     def format_node(self,
-                    placeholder_names: List[str] = None,
-                    maybe_return_typename: List[str] = None) -> Optional[str]:
+                    placeholder_names: Optional[List[str]] = None,
+                    maybe_return_typename: Optional[List[str]] = None) -> Optional[str]:
         """
         Return a descriptive string representation of ``self``.