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
](https://our.intern.facebook.com/intern/diff/17505206/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/26312
Pulled By: driazati
Differential Revision: D17505206
fbshipit-source-id: 1331e412f938e2f08ecb079972147f11e3ec77cd
