Turn the HuggingFaceConceptsLib into a singleton to prevent redundant connections

[lstein]

This is a partial fix for #3330 . It prevents the concepts library from hitting HuggingFace to download concept library terms every time a model is changed.

It does not address the issue that the web interface downloads the concepts even if it is not going to use them.

[psychedelicious]

This is a separate problem from #3330 , see my comments there. Diffusers makes a connection to HF when calling from_pretrained(). It does not close the connection when that command finishes its business, it persists until the python process is ended.

[lstein]

This is a separate problem from #3330 , see my comments there. Diffusers makes a connection to HF when calling from_pretrained(). It does not close the connection when that command finishes its business, it persists until the python process is ended.

Ok. Appreciate the detective work. This PR addresses a different problem then.