Add the x64 msvc toolchain into PATH (pytorch#12446)

Summary:
A possible fix for the problem stated in pytorch#12410.
Pull Request resolved: pytorch#12446

Differential Revision: D10238572

Pulled By: soumith

fbshipit-source-id: 17ade148c4036d2481b878e5cd7d9d67c1e3626e

Author: peterjc123

.jenkins/pytorch/win-build.sh

@@ -91,6 +91,7 @@ if "%REBUILD%"=="" ( call conda install -y -q numpy cffi pyyaml boto3 )
 :: Install ninja
 if "%REBUILD%"=="" ( pip install ninja )
 
+call "C:\\Program Files (x86)\\Microsoft Visual Studio\\2017\\Community\\VC\\Auxiliary\\Build\\vcvarsall.bat" x64
 call "C:\\Program Files (x86)\\Microsoft Visual Studio\\2017\\Community\\VC\\Auxiliary\\Build\\vcvarsall.bat" x86_amd64
 
 git submodule update --init --recursive