[Windows] Workaround for loading bundled DLLs (#4893)

* [Windows] Workaround for loading bundled DLLs

Python-3.8+ adds `add_dll_directory` call, see https://docs.python.org/3/whatsnew/3.8.html#ctypes
Simulate this behaviour on older versions of Python runtime by calling
`LoadLibraryExW` with the appropriate flags

Fixes https://github.com/pytorch/vision/issues/4787

torchvision/extension.py

+import ctypes
+import os
+import sys
+from warnings import warn
+
 import torch
 
 from ._internally_replaced_utils import _get_extension_path
@@ -67,4 +72,22 @@ def _check_cuda_version():
     return _version
 
 
+def _load_library(lib_name):
+    lib_path = _get_extension_path(lib_name)
+    # On Windows Python-3.8+ has `os.add_dll_directory` call,
+    # which is called from _get_extension_path to configure dll search path
+    # Condition below adds a workaround for older versions by
+    # explicitly calling `LoadLibraryExW` with the following flags:
+    #  - LOAD_LIBRARY_SEARCH_DEFAULT_DIRS (0x1000)
+    #  - LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR (0x100)
+    if os.name == "nt" and sys.version_info < (3, 8):
+        _kernel32 = ctypes.WinDLL("kernel32.dll", use_last_error=True)
+        if hasattr(_kernel32, "LoadLibraryExW"):
+            _kernel32.LoadLibraryExW(lib_path, None, 0x00001100)
+        else:
+            warn("LoadLibraryExW is missing in kernel32.dll")
+
+    torch.ops.load_library(lib_path)
+
+
 _check_cuda_version()

torchvision/io/_video_opt.py

@@ -5,12 +5,11 @@ from typing import List, Tuple, Dict, Optional, Union
 
 import torch
 
-from .._internally_replaced_utils import _get_extension_path
+from ..extension import _load_library
 
 
 try:
-    lib_path = _get_extension_path("video_reader")
-    torch.ops.load_library(lib_path)
+    _load_library("video_reader")
     _HAS_VIDEO_OPT = True
 except (ImportError, OSError):
     _HAS_VIDEO_OPT = False

torchvision/io/image.py

 from enum import Enum
+from warnings import warn
 
 import torch
 
-from .._internally_replaced_utils import _get_extension_path
+from ..extension import _load_library
 
 
 try:
-    lib_path = _get_extension_path("image")
-    torch.ops.load_library(lib_path)
-except (ImportError, OSError):
-    pass
+    _load_library("image")
+except (ImportError, OSError) as e:
+    warn(f"Failed to load image Python extension: {e}")
 
 
 class ImageReadMode(Enum):