Remove need of include Python.h in vision.cpp

[fmassa]

🚀 Feature

Related to #2349

We currently need to #include <Python.h> in

vision/torchvision/csrc/vision.cpp

Line 4 in 96ad7f0

#include <Python.h>

as well as we need to define PyInit__C for Windows

vision/torchvision/csrc/vision.cpp

Lines 15 to 22 in 96ad7f0

	// If we are in a Windows environment, we need to define
	// initialization functions for the _custom_ops extension
	#ifdef _WIN32
	PyMODINIT_FUNC PyInit__C(void) {
	// No need to do anything.
	return NULL;
	}
	#endif

This is ideally not needed. Windows builds currently require it due to an artifact on how we build our C++ extensions I believe (with setuptools).

We should try to remove this. This might involve using CMake to compiling the extensions (instead of setuptools), so might involve significant rework.

Further discussion can be found in #3087 (comment)

For reference, this is how TF extensions does it for loading extensions, vs our custom code in

vision/torchvision/extension.py

Lines 8 to 47 in 96ad7f0

	def _register_extensions():
	import os
	import importlib
	import torch
	# load the custom_op_library and register the custom ops
	lib_dir = os.path.dirname(__file__)
	if os.name == 'nt':
	# Register the main torchvision library location on the default DLL path
	import ctypes
	import sys
	kernel32 = ctypes.WinDLL('kernel32.dll', use_last_error=True)
	with_load_library_flags = hasattr(kernel32, 'AddDllDirectory')
	prev_error_mode = kernel32.SetErrorMode(0x0001)
	if with_load_library_flags:
	kernel32.AddDllDirectory.restype = ctypes.c_void_p
	if sys.version_info >= (3, 8):
	os.add_dll_directory(lib_dir)
	elif with_load_library_flags:
	res = kernel32.AddDllDirectory(lib_dir)
	if res is None:
	err = ctypes.WinError(ctypes.get_last_error())
	err.strerror += f' Error adding "{lib_dir}" to the DLL directories.'
	raise err
	kernel32.SetErrorMode(prev_error_mode)
	loader_details = (
	importlib.machinery.ExtensionFileLoader,
	importlib.machinery.EXTENSION_SUFFIXES
	)
	extfinder = importlib.machinery.FileFinder(lib_dir, loader_details)
	ext_specs = extfinder.find_spec("_C")
	if ext_specs is None:
	raise ImportError
	torch.ops.load_library(ext_specs.origin)

(and for image and video as well).

Given that our extensions are now completely independent from Python, we just need to know the location where they live and directly call into torch.ops.load_library.

Note that we will also need to make the solution compatible with fbcode

[fmassa]

@prabhat00155 this might be related to the task I've assigned you. At least the part of the extension loading (hopefully we will not need to change the build, but it's not 100% sure)