Fix visual studio generator (#98605)
If `CMAKE_GENERATOR=Visual Studio 16 2019` then the build will fail if `USE_NINJA=False` not set.
This PR changes that if CMAKE_GENERATOR is set an not equal to ninja then it won't use Ninja.
This is just for easier setting another generator.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/98605
Approved by: https://github.com/kit1980
