python interop for script classes (#18148)

python interop for script classes (#18148)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18148
ghimport-source-id: 40a9d745dc9aeba53d098743323fcbd50ca65137

Stack from [ghstack](https://github.com/ezyang/ghstack):
* **#18148 py interop**

Support for converting classes between the Python–TorchScript boundary. Like other TorchScript values, ScriptClasses are native Python values when used in Python and IValues when used in TorchScript.

Notably, there is a copy across this boundary, which will be surprising to users who will expect standard Python reference semantics. I have some ideas for fixing that, but it's a more involved process.

Reviewed By: jamesr66a

Differential Revision: D14526259

fbshipit-source-id: 5916e3032488a42dc7da756c1826d7c040a21ebd