Windows local build: restore original working dir after activating VC environment...

Windows local build: restore original working dir after activating VC environment (#14416)

Summary:
`call "C:\\Program Files (x86)\\Microsoft Visual Studio\\2017\\Community\\VC\\Auxiliary\\Build\\vcvarsall.bat" x64` seems to change the working dir to `C:\Users\Administrator\source`, and we need to cd back to the PyTorch directory before running `git submodule update --init --recursive`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/14416

Differential Revision: D13222269

Pulled By: yf225

fbshipit-source-id: a0eb3311fb11713b1bb8f52cd13e2c21d5ca9c7b