Add `clone_instance` for Module (#30168)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/30168
Previous implementation of `clone` in `script::Module` copies both the module instance and the
class type, after we enabled type sharing https://github.com/pytorch/pytorch/pull/26666 we also
need to have a function to clone instance only and share the underlying class type.
Test Plan:
tbd
Imported from OSS
Differential Revision: D18631324
fbshipit-source-id: dbadcf19695faee0f755f45093b24618c047b9d1
