Update cpp_extension.py (#18638)

Update cpp_extension.py (#18638)

Summary:
Hi. It seems that when building CPP-extensions with CUDA for Windows, an `extra_cuda_cflags` options are not properly forwarded to `nvcc`.

Use of extra CUDA options is necessary to build, for instance, a InplaceABN (https://github.com/mapillary/inplace_abn), which requires `--expt-extended-lambda` option.

This PR adds one line that correctly appends `extra_cuda_cflags`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18638

Differential Revision: D14704270

Pulled By: ezyang

fbshipit-source-id: e1e330d193d9afd5707a5437a74c0499460d2b90