Add `Final[T]` annotated members to `__constants__` (#21603)
Summary:
Class member annotations can be marked with `Final[T]` instead of adding them to `__constants__`. `Final` comes from the `typing_extensions` module (which will be used if it is present). If not, the polyfill from `_jit_internal` is exposed as `torch.jit.Final` for users that don't want to install `typing_extensions`.
This keeps around `__constants__` since a lot of code is still using it, but in documentation follow ups we should change the examples to all to use `Final`.
TODO: install typing_extensions on CI, move tests to a Python3 only file when #21489 lands
](https://our.intern.facebook.com/intern/diff/15746274/)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21603
Pulled By: driazati
Differential Revision: D15746274
fbshipit-source-id: d2c9b5643b4abba069b130c26fd42714c906ffac
