Add langchain embedder

[regaltsui]

What does this PR do?

This PR adds a Langchain Embeddings class as the backend of BERTopic. The Langchain Backend supports LangChain compatable Embeddings, including 3rd party integration.

Github Issue

#2293

Before submitting

• This PR fixes a typo or improves the docs (if yes, ignore all other checks!).
• Did you read the contributor guideline?
• Was this discussed/approved via a Github issue? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes (if applicable)?
• Did you write any new necessary tests?

[MaartenGr]

Thanks for the PR!

There was an odd error with the GH-workflow, so I reran the checks. That said, I think we can skip adding LangChain to the pyproject.toml for two reasons. First, there are no tests, so I cannot guarantee that a given version will work. Second, considering the LangChain would have you install additional dependencies anyway (depending on the embedding model), doing pip install bertopic[langchain] wouldn't be enough.

[regaltsui]

Sure, I have just removed all changes on the pyproject.toml. Let's see how the GH-workflow goes

[MaartenGr]

Awesome, it seems that it is working well. Would you mind adding a small section to the documentation? Above flair would be nice considering most users would want to use sbert/transformers first. It doesn't have to be much, so just the docstrings and a few lines to explain what LangChain is and where to find/install it.

[MaartenGr]

Thanks for adding the docs, after the conflicts have been resolved and the tests pass then I'll go ahead and merge this.

[regaltsui]

I have just rebased the branch as I see FastEmbed has been added recently. Let's see if it's passing the tests.

[MaartenGr]

Awesome, thank you for your thoughtful work on this and your quick replies. Great to have this in BERTopic.