Add qscheme() method (#20608)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/20608
Exposing QScheme in python as Python objects like `torch.qscheme.per_tensor_affine` etc.
Reviewed By: zafartahirov
Differential Revision: D15364354
fbshipit-source-id: 4d6a96d67e9ead051cf4a8f934553a8c7232fdb7
