I’ve changed my setup.py to use the CppExtension:
from distutils.core import setup, Extension
from torch.utils.cpp_extension import BuildExtension, CppExtension

def configuration(parent_package='', top_path=None):
      import numpy
      from numpy.distutils.misc_util import Configuration
      from numpy.distutils.misc_util import get_info

      #Necessary for the half-float d-type.
      info = get_info('npymath')

      config = Configuration('',
                             parent_package,
                             top_path)

      config.ext_modules.append(CppExtension(
                name='reson8',
                sources=['reson8.cpp'],
                extra_info=info,
                extra_compile_args=['-g'],
                include_dirs=["/home/ian/anaconda3/lib/python3.7/site-packages/pybind11/include",
                                          "/home/ian/anaconda3/lib/python3.8/site-packages/pybind11/include",
                                          "/home/ian/dev/hedgey/Engine/lib/libtorch/include",
                                          "/home/ian/dev/hedgey/Engine/lib/libtorch/include/torch/csrc/api/include"]
                                ))

      return config


if __name__ == "__main__":
      from numpy.distutils.core import setup
      setup(configuration=configuration)

But I get a similar error:
ImportError: reson8.so: undefined symbol: _ZTIN3c1021AutogradMetaInterfaceE


So there’s still a build problem…