Move everything model-/source-specific to custom model file or source-specific extractor

[LukasMut]

Currently, we have some if-statements that address model-specific exceptions. Since these are exceptions rather than something general, we want to specify them in the custom model file or move them to source-specific extractor classes, if applicable. There's an if-statement that exclusively concerns clip models. This should go into the custom model file or the extractor classes for OpenCLIP.

[LukasMut]

@andropar Correct me if I am wrong, but I think you, Jonas and I fixed that issue in different PRs.

[andropar]

I think this is also related to the custom methods defined here: https://github.com/ViCCo-Group/thingsvision/blob/master/thingsvision/core/extraction/helpers.py#L58

[LukasMut]

@andropar Right. Then, this is still an open issue.