Windows nvtx for CUDA 11.6 require _wgetenv decl (#79798)
Summary:
The include ordering modified via clang-format prevented a
declaration of _wgetenv, declared and used in nvtxInit.h, which was
previously acquired transitively through the pybind include. Provide an
explicit platform include of wchar.h which defines it on Windows, prior
to including nvToolsExt.h
Test Plan: Windows msvc cuda11 build should pass.
Differential Revision: D37244689
Pull Request resolved: https://github.com/pytorch/pytorch/pull/79798
Approved by: https://github.com/kit1980
