[Windows] Workaround for loading bundled DLLs

[malfet]

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 #4787

cc @peterjc123 @nbcsm @guyang3532 @maxluk @gunandrose4u @mszhanyi

[facebook-github-bot]

💊 CI failures summary and remediations

As of commit cbf01c9 (more details on the Dr. CI page):

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• 2/2 failures introduced in this PR

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2 failures not recognized by patterns:

Job	Step	Action
python_type_check	sudo apt-get update -y
sudo apt install -y libturbojpeg-dev
pip install --user --progress-bar off mypy
pip install --user --progress-bar off types-requests
pip install --user --progress-bar off --pre torch -f https://download.pytorch.org/whl/nightly/cpu/torch_nightly.html
pip install --user --progress-bar off git+https://github.com/pytorch/data.git
pip install --user --progress-bar off --no-build-isolation --editable .
mypy --config-file mypy.ini
	🔁 rerun
binary_libtorchvision_ops_android	Build	🔁 rerun

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[pmeier]

I can't comment on the actual content. One minor syntax error and a few questions inline.

[andfoy]

It seems that these changes were also introduced back in PR #2811, do we know why they were removed?

[malfet]

It seems that these changes were also introduced back in PR #2811, do we know why they were removed?

Hmm, that's a very good point. Looks like it was accidentally deleted by #4080
cc: @NicolasHug

[NicolasHug]

It seems that these changes were also introduced back in PR #2811, do we know why they were removed?

Hmm, that's a very good point. Looks like it was accidentally deleted by #4080 cc: @NicolasHug

Thanks for the ping, and sorry for the trouble. Prior to #4080 we had 3 different registration mechanisms - one for extensions.py, one for image, and one for video. I unified them into a single one - it wasn't clear why each of them were different (no comments, no link to related issues), so I picked the simplest one that seemed to work: the one from extension.py.

[fmassa]

Thanks a lot Nikita, and sorry for the delay in getting back on this.

I have one high-level comment, otherwise the PR is good to merge.

[malfet]

Moved _load_library to torchvision.extension

[fmassa]

nit: you probably need to move those imports to extension.py now

[github-actions]

Hey @malfet!

You merged this PR, but no labels were added. The list of valid labels is available at https://github.com/pytorch/vision/blob/main/.github/process_commit.py

[adamjstewart]

Is there a reason that this was changed from pass to warn? When running pytest on my project, I see warnings like:

../.spack/.spack-env/view/lib/python3.9/site-packages/torchvision/io/image.py:11
  /Users/Adam/.spack/.spack-env/view/lib/python3.9/site-packages/torchvision/io/image.py:11: UserWarning: Failed to load image Python extension: 
    warn(f"Failed to load image Python extension: {e}")

It's unclear if this is a serious bug in my installation that I should fix, or something I can safely ignore. FWIW, I'm using the pillow-SIMD backend.