do not use constexpr with CUDA >= 9.2 compiler on Windows. (#18986)

do not use constexpr with CUDA >= 9.2 compiler on Windows. (#18986)

Summary:
Define `AT_CPP14_CONSTEXPR` from `constexpr` to empty on Windows with CUDA >= 9.2 as workaround.

Discussed in #18425.

When using CUDA 10.1 on Windows, I faced following errors:
~~~
D:/data/source/pytorch\c10/util/ArrayRef.h(144): error: variable in constexpr function does not have automatic storage duration
          detected during instantiation of "const T &c10::ArrayRef<T>::front() const [with T=at::Tensor]"
D:/data/source/pytorch/aten/src\ATen/DeviceGuard.h(30): here
~~~

From documentation of CUDA Toolkit v10.1.105, compiler supports `constexpr` and relaxing requirements (in C++14), but compilation failed.

I suppose this could be compiler bug and require this workaround.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18986

Differential Revision: D14821836

Pulled By: ezyang

fbshipit-source-id: 9800da2fe7291e7c09e8e5e882adebab08d83ae3