[WIP] 🚨 clean xcodec 🧼

[eustlb]

What does this PR do?

Lots of stuff to clean on Xcodec integration to reach the source of truth expected quality, mainly:

1. Stuff could have been simplified a lot, the current integration is mainly a dump from the original codebase, and modular should have been used!
2. The semantic decoder is instantiated but not used since we do not support training (and same for it's proj layer), adding dead weight to the model. Either we use it and enable training, either we remove it. No big interest in supporting training for now → let's remove
3. The two passes on the encoder only to get the ouput shape causes mem spikes, it is uncessary!

Warning

This PR is breaking. TODO: let's see how to reduce the arm for users...

[github-actions]

[For maintainers] Suggested jobs to run (before merge)

run-slow: dac, xcodec