add type refinements for isinstance checks (#27772)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/27772
This replaces unchecked_unwrap_optional with unchecked_cast. This
enables the generalization of type refinement so that it works for
isinstance checks as well. This also removes unchecked_unwrap_optional from
code we generate, which is good because it is a hard op to serialize well
since it doesn't directly encode the Optional[T] being unwrapped. In contrast,
unchecked_cast always explicitly lists the type.
Test Plan: Imported from OSS
Differential Revision: D17885424
Pulled By: zdevito
fbshipit-source-id: ce81077d6fbeaf2a802a2e0b17349aca85670466
