Fix load_torchcodec_shared_libraries on Windows

[aboood40091]

Add support for locating FFmpeg DLLs on Windows.

Fixes #1108, #1014, and several other issues.

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[NicolasHug]

Thanks @aboood40091

I'm happy to try that out if CI is green. It's very hard to debug Windows issues for us because we don't have easy access to Windows machines, so I do appreciate the help on this.

What I do not understand though: why are users experiencing these issues, when our Windows CI jobs are just fine? There must be a fundamental env difference that I'm currently missing?

[NicolasHug]

I did a quick search and I wasn't able to find an authoritative source on what a full-shared version is. I assume it's an version that ships dll instead of statically linking all libraries.

My concern is that this error message may not be directly actionable for users, unless we add more details on how they can install such a "full-shared" version. These details aren't really in scope here, so maybe we can leave this error message as it was?

[aboood40091]

Correct. "full-shared" is the version that ships DLLs instead of statically linking them. "full-shared" is the term used in the download page.

Personally, I did not know the shared version is the one that has all DLLs until I started digging enough, so I figured this information would be helpful.

If you still would like me to reverse the change to the message, please let me know.

[aboood40091]

Thanks @aboood40091

...

What I do not understand though: why are users experiencing these issues, when our Windows CI jobs are just fine? There must be a fundamental env difference that I'm currently missing?

Personally, I don't know. It actually should not work for you under normal circumstances according to the Python documentation due to a breaking change in Python 3.8. Refer to:
https://docs.python.org/3/whatsnew/3.8.html#bpo-36085-whatsnew

The use of add_dll_directory is required now as PATH will no longer be searched for DLLs. Perhaps whatever environment you have somehow overrides this, or it's already called add_dll_directory somewhere behind the scenes.

[NicolasHug]

Thanks for the PR @aboood40091 . CI is failing with an unrelated issue which is also visible in #1111. I'll try to fix that first, then hopefully this PR will be green and I'll merge it.

[Dan-Flores]

The two error signals are known and being tracked in #1047, lets merge this PR. Thanks!

[aboood40091]

@NicolasHug in your latest changes, cm is undefined if ffmpeg path is null. Is this intentional?

[NicolasHug]

😮 thanks for catching this @aboood40091 . Let me submit a follow-up. We're planning to publish a bugfix release later today with your windows fix.