Rework weight conversions

[Laurent2916]

That's a pretty fat PR:

• Deleted all old conversion scripts.
• Created a new conversion module which converts weight state_dicts based on simple mappings (most of the time), inspired by imaginAIry.
  • Mapping conversion is way more lighter/faster than using the old ModelConverter (this allows weights to be converted on the fly if necessary/wanted).
  • Each conversion is now declarative. The hash (and sometime git commit revision) of both the original weights and converted weights are now strictly checked (to avoid unwanted regressions).
• Created new test fixture use_local_weights that allows to run the tests either with local converted weights, or the ones from the refiners huggingface org.
• Modified all existing tests to use these new fixtures.
• Added DINOv2 similarity when testing images in e2e tests.
• Loosen some tests to be good on both a 3090 and an L4.

See the updated CONTRIBUTING.md for instructions.