Expose Demuxer, PacketDecoder and ColorConverter

[NicolasHug]

First of many PRs to expose lower-level video decoding blocks to let users overlap stages. For example:

Method                             Time (ms)     CPU %   vs VideoDecoder
------------------------------------------------------------------------
VideoDecoder                  864.4 +/- 12.0       100             1.00x
sequential                     748.8 +/- 2.2       100             1.15x
demux+decode || cc             537.1 +/- 5.6       121             1.61x
demux || decode+cc             754.4 +/- 2.0       102             1.15x
demux || decode || cc          533.9 +/- 8.3       126             1.62x

CPU-only for now. Lots of TODOs and tons of internals / public stuff will change. This is a big PR, but it barely changes any existing code, so it's safe to merge.

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/meta-pytorch/torchcodec/1501

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