[torchvision] Move video utilities to internal fb/io location (#9433) (#9433)

[NicolasHug]

Summary:
This diff moves the torchvision.io video utilities (read_video, write_video, read_video_timestamps) from the OSS location to the internal fb/io location.

Changes:

• Created fb/io/video.py with the full implementation of video functions
• Updated fb/io/init.py to export video functions and internal helpers
• Updated torchvision/io/init.py to import video functions from fb/io
• Made torchvision/io/video.py a stub that re-exports from fb/io/video.py
• Removed video functions from torchvision.io.all (for OSS)
• The (rare) TorchVision stuff that depended on video decoding utils have been migrated to TorchCodec (that's for TV only, not for the rest of the internal users).

This change ensures:

• Internal users can continue using 'from torchvision.io import read_video'
• Internal users importing from torchvision.io.video directly still work
• No BUCK changes are required
• OSS/GitHub users will no longer have access to these deprecated video APIs

Eventually we'll want to migrate all these internal users to TorchCodec, but that's for later.

Note: in D95081771 I bluntly removed all the torchvision decoder stuff from both GH and fbcode, but that ended up leading to tons of failed tests (that weren't triggered on the diff!). This new diff is a softer version of that.

Differential Revision: D95933713

Pulled By: NicolasHug

[pytorch-bot]

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