Update `README.md` in oss (#43893)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/43893
Update `README.md` in oss, provide more examples, start from the most common use to specified use. Make `README.md` be more friendly and more specific.
Test Plan: `README.md` doesn't need test.
Reviewed By: malfet, seemethere
Differential Revision: D23420203
fbshipit-source-id: 1a4c146393fbcaf2893321e7892740edf5d0c248
Author
yujunzhao@devvm1621.atn0.facebook.com
