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Desribe the bug

Title pretty much says it all.

BERTopic/bertopic/backend/_sentencetransformers.py:4
    from sentence_transformers.models import StaticEmbedding

Looks like this is an issue with versions. SentenceTransformer didn't add Model2Vec till
#2245
but there's no requirement that the new version of SentenceTransformer is installed.

I'll submit a PR to fix this, but I don't know what solution you'd prefer. I think the best option is to move that import from line 4 to line 58, since it appears that's the only place it's needed. I'll make a PR and test it.

Reproduction

from bertopic.backend._sentencetransformers import SentenceTransformerBackend

BERTopic Version

v0.17.0