Fix mypy error (#48359)
Summary:
Fixes error introduced in https://github.com/pytorch/pytorch/pull/47657
`node.target` can be either a str or a callable, but this is checked in the pattern matching portion.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/48359
Reviewed By: ilia-cher
Differential Revision: D25141885
Pulled By: Chillee
fbshipit-source-id: 94365a5a3dd351652ea7337077cd0e71b6ffe203
