.circleci: Add simple backup and restore solution for RCs (#38690)
Summary:
* Does a basic upload of release candidates to an extra folder within our
S3 bucket.
* Refactors AWS promotion to allow for easier development of restoration
of backups
Backup restoration usage:
```
RESTORE_FROM=v1.6.0-rc3 restore-backup.sh
```
Requires:
* AWS credentials to upload / download stuff
* Anaconda credentials to upload
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38690
Differential Revision: D21691033
Pulled By: seemethere
fbshipit-source-id: 31118814db1ca701c55a3cb0bc32caa1e77a833d
