Make `is_optional` check more robust (#26312)
Summary:
If the `Union` contains a non-class type, `issubclass` would fail, this
adds a check for that case
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26312
Pulled By: driazati
Differential Revision: D17486465
fbshipit-source-id: c513cef3bbc038f15c021eb0c1bf36be0df1eb90
