Improve build docs and process for Windows (#21190)
Summary:
Fixes #21026.
1. Improve build docs for Windows
2. Change `BUILD_SHARED_LIBS=ON` for Caffe2 local builds
3. Change to out-source builds for LibTorch and Caffe2 (transferred to #21452)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21190
Differential Revision: D15695223
Pulled By: ezyang
fbshipit-source-id: 0ad69d7553a40fe627582c8e0dcf655f6f63bfdf
