Update _bertopic.py to address #2316

[nhansendev]

Added "embeddings" kwarg to update_topics for use when pre-calculated document embeddings are used. Addresses issue #2316

What does this PR do?

Fixes #2316

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[nhansendev]

The user could easily miss that they need to provide the embeddings to update_topics. Should there be some sort of warning? Maybe a check for whether "embeddings" and "self.embedding_model" are both None?

[MaartenGr]

Might indeed be nice to have such a check here but I'm not sure whether it would cover all cases considering the _create_topic_vectors also has a few different options for creating topic vectors.

[MaartenGr]

I'm rerunning all jobs since the error didn't relate to your code but a connection issue with huggingface.