Expose QScheme in frontend (#19381)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19381
Expose QScheme enum in frontend so that people can use it in
quantization configs in modules.
Differential Revision: D14922992
fbshipit-source-id: ab07b8a7ec42c1c1f5fe84a4a0c805adbcad408d
