Model registry for special use cases

[blumenstiel]

Is your feature request related to a problem? Please describe.
We have models that can be directly applied for specific applications such as image reconstruction, compression or de-tokenization. It is unclear where we register these models.

Describe the solution you'd like
We could either register all these models in the BACKBONE_REGISTRY or created a new registry for these special use cases. Viewing them as backbones is not ideal as some cannot be used as backbones. Also, labeling a full MAE model as backbone is also not correct. Having another registry might be more complex but it also clearly differentiate between models suited for backbones and other models.

@romeokienzler @paolo-fraccaro What do you think? If we add one, how should we call this special registry?