Closes #3965

For some reason, on Windows we need to define the PyInit_video_reader PyInit_image and PyInit__C symbols, otherwise the windows linker fails with something like

LINK : error LNK2001: unresolved external symbol PyInit__C
%SRC_DIR%\build\temp.win-amd64-cpython-38\Release\Jenkins\Miniconda3\conda-bld\torchvision_1715167042718\work\torchvision\csrc\ops\autocast\_C.lib : fatal error LNK1120: 1 unresolved externals

To avoid that we were defining the symbols as

PyMODINIT_FUNC PyInit__C(void) {
  // No need to do anything.
  return nullptr;
}

But that PyMODINIT_FUNC forced us to include Python.h and take a dependency on Libpython. (BTW, we probably never needed to link to libpython like we did in the CMakeFile.txt, we probably only needed the headers, but that's another story.)

This PR changes this PyMODINIT_FUNC into a void*, which seems to work. In reality, PyMODINIT_FUNC is PyObject_t*.