Fix external DLL loading on wheels

[andfoy]

This PR registers the path of the main torchvision library via AddDllDirectory on Python < 3.7 and os.add_dll_directory on Python >= 3.8. The extensions are preloaded using LoadLibraryExW to circunvent the limitation imposed by AddDllDirectory without calling SetDefaultDllDirectories

[fmassa]

Thanks for the PR!

I'm not very comfortable with the current solution.

Would there be a way to fix Windows without having to do this PATH manipulation at import time?

For reference, we identified in #2775 that the Windows pip package for torchvision are still not loading image.dll correctly, and this PR is a tentative fix for it.

[codecov]

Codecov Report

Merging #2811 into master will decrease coverage by 0.07%.
The diff coverage is 62.00%.

@@            Coverage Diff             @@
##           master    #2811      +/-   ##
==========================================
- Coverage   73.33%   73.26%   -0.08%     
==========================================
  Files          99       99              
  Lines        8730     8778      +48     
  Branches     1374     1387      +13     
==========================================
+ Hits         6402     6431      +29     
- Misses       1909     1920      +11     
- Partials      419      427       +8     

Impacted Files	Coverage Δ
torchvision/io/_video_opt.py	40.57% <56.25%> (+1.19%)	⬆️
torchvision/extension.py	67.21% <64.70%> (-0.97%)	⬇️
torchvision/io/image.py	82.25% <64.70%> (-6.88%)	⬇️

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[andfoy]

It seems that this solution is damaging also the binary imports of other libraries, such as Scipy

[fmassa]

@peterjc123 could you help review this PR?

For context, we identified in #2775 that the Windows pip package for torchvision are still not loading image.dll correctly, and this PR is a tentative fix for it.

[peterjc123]

LGTM. BTW, it would be better to refactor the code to eliminate duplicates.

[peterjc123]

Would there be a way to fix Windows without having to do this PATH manipulation at import time?

No, unlike unix platforms, Windows doesn't have an alternative for RPATH for native executables.

[fmassa]

Thanks @andfoy and @peterjc123 , I'm merging this to unblock and do some testings.

Also, one thing that I would like to point out is that there is a non-symmetric inclusion here, where we add the path to the DLLs in extension.py but not in image and video, which might add the implicit requirement that we need to import extension.py before the others. Not sure if this is a problem?