Add stubs or py.typed

[urucoder]

Add missing stubs or py.typed. Following the documentation at CONTRIBUTING.md#type-hints and the checking in the code I can see that the library has type hints, but when running Mypy checker it fails with the error

Skipping analyzing "opacus": module is installed, but missing library stubs or py.typed marker [import-untyped]

So I'm hoping if either stubs or a py.typed file can be added to solve this, thanks.

[HuanyuZhang]

Let me take a look and get back to you