fixes _get_embeddings for ChromaDB

[lukaskuhn-lku]

As described in #389 _get_embeddings is throwing an error when called via retriever(query): TypeError: Embeddings.create() got an unexpected keyword argument 'engine'.

This is a result of the unnecessary usage of model_args and api_provider. I removed both and now the function works as expected.

[insop]

@lukaskuhn-lku, it seems like https://github.com/stanfordnlp/dspy/pull/400 update has wider scope changes. Maybe this PR might not be necessary if the newer one (400) is reviewed and merged.

[okhat]

Thank you so much @lukaskuhn-lku ! I will merge #400 so I will close this, but I really appreciate your contribution.