Decompose more Windows scripts (#18917)
Summary:
This PR:
* pulls four distinct installation steps out of `build_pytorch.bat` and into their own scripts.
* eliminates the copy step for helper scripts called by `win-build.sh` and `win-test.sh`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18917
Differential Revision: D14807236
Pulled By: kostmo
fbshipit-source-id: 03e91a5834dfd6d68903ad9725eacc099bbf6d53
