Fixes caffe2 loading issues on Windows (#39513)
Summary:
Addresses https://github.com/pytorch/pytorch/issues/27840#issuecomment-638715422.
Contains a bunch of fixes (https://github.com/pytorch/pytorch/pull/39376 + https://github.com/pytorch/pytorch/pull/39334 + https://github.com/pytorch/pytorch/pull/38302 + https://github.com/pytorch/pytorch/pull/35362)
Pull Request resolved: https://github.com/pytorch/pytorch/pull/39513
Differential Revision: D22190761
Pulled By: malfet
fbshipit-source-id: b2d52f6cb16c233d16071e9c0670dfff7da2710e
