Python version agnostic C++ extensions

[zou3519]

Recording some offline discussions with @drisspg and @malfet and also because this came up again in a thread on Slack

Today, C++ extensions in PyTorch need a separate build per python version. This is for two main reasons:

1. they link against libtorch-python, which has a separate build per python version
2. Python C extensions assume that you need to link against cpython, which invoves a separate build per python version.

Pitch: for C++/CUDA custom-operator only C++ extensions (which is the vast majority of them), if we can excise all the python dependencies, then one should be able to have a single build that works for all Python versions.

cc @malfet @ezyang @msaroufim @bdhirsh @anijain2305 @chauhang

[malfet]

This is already a default behavior, isn't it?